Sunday, February 17, 2019

Delvers of Dàrkmesa - Campaign Rules

Forward: Delvers of Dàrkmesa

The Delvers of Dàrkmesa campaign rules represent the culmination of my ten years exploring the theory and spirit of tabletop roleplaying games in the vein of Dungeons & Dragons.

This project began in 2008 in the wake of D&D 4th Edition. But that inflection point needs some context.

I had been playing D&D since 2003, when a new friend gave me a copy of the 3rd Edition D&D Player's Handbook, having recently upgraded to 3.5 himself. I read every word in that handbook one thousand times over. I was running games of 3.5 for my friends soon after, and quickly settled into a nice groove: each campaign I tried to run would last three to four sessions, only to spiral out of control due to my lack of experience.

In my heart, I usually blamed the players for how things would fall apart. I had played the epic campaigns through in my head, and tried to run them all to that end. Didn't they know that Tolkien-level excitement and wonder awaited them, if they could just go along with the story? They never seemed to march to the tune, despite their best efforts. Luckily, we were young, and all still in school in our home town. Starting a new game every month was never an issue. I had a million tired fantasy narrative tropes that I yearned to emulate at the table, and a good handful of friends more than willing to indulge me.

I never admitted that I blamed them, and that was good, because it was my fault and I didn't know it. I didn't know that the stories I liked to read and write were not the ones I liked to play in or run. I couldn't see that my campaigns as conceived were little more than setting and stage direction. It was truly the bliss (or perhaps, frustration) of ignorance.

You see, the friend that introduced me to D&D was the first person I ever met who played. That made me the second person I ever knew who played. Each of my other friends who I invited to play then became the third, fourth, fifth, and so on. We were making it up as we went. Sure, we were using the rules. But the rules don't tell you how to play. Not really. And back then, it was faster to walk to the library and check-out a Dragonlance novel than to look up anything online. How other people played was a complete mystery to all of us.

Around 2007, the older cousin of a friend of mine came to town. His name was (is) Nick, and besides being a math genius, he was also an avid player of 2nd edition D&D. Once he found out his little cousin and co were all playing D&D on the weekends, he offered to run a (3.5 converted) game of Ravenloft for seven or eight of us.

That was good D&D. I played a monk, which was a terrible class in 3.5. But the D&D was good, so I had fun.

Then we went back to playing our own games (which I ran), and they sucked again. I would often pine away in my memory for the details of that game, searching for its secret sauce.

In 2008, 4th edition D&D is released, and for the first time, the veil of design is peeled back for me. The deliberate ways that the game was laid out and organized, the brutal crackdown on the numbers, the every inch regulated and standardized.

D&D it was not. My friends all lamented at the something that was lost in translation. I maintained always that I loved 4th edition, but found it rather hard to express precisely why in the language of fun. In my eyes, 4th edition was designed to a very specific end: tactical combat play. In that, it succeeded. It was designed to be learned quickly. Again, it succeeded there. It was designed to look modern and aesthetically consistent. A+ again. It was the right product, but for the wrong market.

So my theory crafting and homebrewing began. "There must be a way," I thought, "to bring the principles of design from 4th edition into 3.5 such that we can play a superior hybrid game."

It should be said: 3.5 was universally regarded as broken by 2008. How broken exactly was up for debate, but the sheer lack of direction for the design of the game and its splat books was becoming a malignant issue. There was too much game, and no central idea to measure each component's worth and validity against. One of the primary draws to a new game from the player's perspective was the chance to try out new classes and options. But from the other side of the table, every new feature was a chance to break the game.

Notebook after notebook did I fill with my Frankenstein D&D hacks. If I recall correctly, the first things I ever scribbled house rules about were skills and feats. Did we really need over 36 skills? And who knows how many hundred feats? Do those options really enrich play, or do they just tickle the fancy of min/maxers?

By 2009, my family had DSL and I had my own laptop for school. So I decided to start Googling these questions and delving for answers in the dark halls of the interweb.

What I discovered was called the Old School Renaissance. It had begun around 2006 or 2007, as best I can tell, with some bloggers connecting on various forums and posts. They shared ideas from their history with the game, which stretched back to its very beginnings, and even before. They knew the old rulesets weren't perfect, and offered their solutions to the age-old problems. There was debate and proselytism among great grognard philosophers, and countless quiet members of the community, like me, just trying to absorb it all.

These champions of the old school tore my fragile conceptions about D&D apart, like the Twelve prophets of the Hebrew Bible chastising Israel. What was this game I had been playing all these years? Where had it come from, and what great pillars of its history had been lost to its many transmutations? Had we taken the wisdom of its forefathers for granted? Nowhere else have I seen such thoughtful effort by people to justify their ideas about a hobby and its function. While everyone else spent three years learning Pathfinder, I was mesmerized by the mysteries of OSR games.

If this era (c. 2006-2011) was indeed the Old School "Renaissance," then what happened in 2012 was the Old School "Industrial Revolution." In January, Wizards of the Coast announced that the short-lived 4th edition would soon be replaced by a new D&D, "D&D Next." In May, they began a massive open playtest of the rules they were working on.

The forums that existed on the old WotC website containing all of the playtest feedback and discussion were truly a treasure of knowledge and design theory. I was well into my obsession with writing a better ruleset by 2012, and I consider myself extremely lucky to have had the chance to explore those million musings before they were archived later that year.

For two years, there seemed to be no word of the next D&D, but my mind never stopped swirling around the problems the OSR had illuminated. As I got further into college, time for playing the game became scarce, but my interest in thinking about it only grew. The homebrews I made became smaller and simpler. There were fewer and fewer rules each time I would revise or start again. I was getting better at the design, but deep down I still didn't understand what I wanted. I was designing for better design's sake, but not for the sake of better play. Indeed, most of my homebrews never saw the light of day. The ones I did test largely felt like playing any other ruleset.

It doesn't matter much what font you write the stage directions in, turns out.

In 2014, D&D 5 came out, and my friends loved it. It was the greatest thing since sliced bread, fixing all the problems with 3.5, but still holding onto that something that 4 had abandoned.

But when I looked at it, I was disappointed. Sure, advantage/disadvantage is a great mechanic. And yeah, the monk is way better now. And oh hey, they are consciously keeping the official splat books and feats and skills and classes under control. All good things, no doubt, for those of us content with the quality of play we had achieved throughout the years. But D&D 5 didn't do what I desperately wanted it to do.

It didn't show me how to design toward something. D&D 5 is the everyman's game. It's a story game, it's a tactical combat game, it's epic high fantasy, it's gritty low fantasy. It's everything, and nothing. At the table, it feels just like 3.5 did when my friends and I couldn't yet afford the extra books.

Fast-forward another year, and Critical Role is D&D's technological revolution. D&D is sexy now, and it rivals the best TV shows for hours of content and number of dedicated fans. Like all instances of celebrity art, Critical Role is at first inspiring, and then soul crushing. Inspiring because it shows you what tabletop RPGs can deliver, and soul crushing because I am not friends with a cadre professional voice actors who want nothing more than to practice their acting around my narrative, with some combat thrown in to spice things up. It isn't just an unattainable ideal, it may actually be an un-approachable ideal depending on the friends you have.

A cursory scan of D&D subreddits these days will show you the Critical Role heartbreak phenomenon writ large. DMs are afraid to let their players fail. They're afraid to change the plans they have for their narrative in response to what the players do. They hand their players stage directions, and instead of amazing voice actors and improv artists, it turns out their players are just regular people. The story-based D&D facade all comes crashing down when you try to fit it into the real context that the game has in our lives. A weekly hobby for a handful of non-writers and non-actors with shifting schedules and more important things to remember than the evil motivations of the Demon Vicar of Nowheresville in your tabletop soap opera guest-starring the player characters.

But even a few years into playing and DMing 5e, I still maintained that top-down DM fiat narrative was a viable play strategy. And it wasn't until a little later, when one of my close friends ran a short-lived game in which I played, that the final trigger for my OSR metamorphosis would occur.

The setup was this: the party (which had several new members) were asked to convene at an inn to receive a quest from an NPC. We did, and the NPC told us how to get to a mythic dungeon full of (literally) civilizations worth of treasure. If we do it, the NPC gets to keep whatever scrolls and spellbooks we find in the loot. My guy is a fighter, so that is fine with me.

The party leaves for the dungeon, which is in the center of a crazy necrotic swamp that has emerged around it because the dungeon is the fossilized remains of an ancient dragon god (cool) which is causing the land to transform as it decays (super cool). When we arrive, there is a crew of evil dudes camping around the dungeon entrance, which we battle and defeat in a fight (sweet). Then, upon entering the dungeon, we find out that all of the treasure has already been looted and thrown through a portal into the plane of Limbo (...what?).

Now, from my friend's perspective, he was offering the potential for a sweet adventure on a rarely-explored plane of existence. And had that been the hook for the adventure from the beginning, I would have been more than excited.

But from my perspective, my character's motivation to adventure had been flushed down the drain. That might not be so bad (hey, things don't always turn out the way your character wants), but my poor dwarf was also trapped by the swamp. I tried to make a choice about what adventure my character would pursue, and then when that was turned on its head, I was denied a second choice. Not only was the original intent of the adventure no longer an option, but if my character jumped through that portal, there was literally no way for me to control whether I could turn back. No way for me to make an actual choice.

I was outraged after the fact, and it surprised me! This was no different than a hundred other times that one of my friends or I had weaved a D&D web around our players to get them to do what we wanted. The difference this time was how obvious it was to me given the mindset that I was in. I didn't blame my friend, I blamed our collective understanding of the game.

The swamp was not an obstacle to the treasure, because the treasure wasn't there anymore. The swamp was an obstacle to beginning the adventure, or any other adventure for that matter, if the party had changed its mind. The dungeon was the single source of player-DM agreement ahead of time, and it was thrown out. The foundational structure of the game, that there was a treasure to be had if some obstacles were overcome, was abandoned for the sake of an unsure reward (the treasure was in Limbo, after all) after an indeterminate amount of time and obstacles.

The contract of the game was broken, I was mad, and that was the answer I had always been looking for. What was the contract that my games should make between the DM and the players? Whatever that contract, design the game toward it.

D&D 5 makes no contracts. All forms of play are equally valid, and therefore there can be no measure of what games succeed or fail except on the scale of fun. This makes DMing, and DM prep especially, a nightmare of guesswork no matter how many books or tips or tools you have. What type of game will be fulfilling? What type of game will last? How do I get the players to ask about my availability to play rather than trying to chase them?

Well what if, given the wealth of OSR knowledge that I had absorbed, I made the contract of my campaign specific in the way that is time tested:

In this campaign, the player characters will explore a megadungeon for treasure. They will frequently return to the outlying town or towns to restock and rest, and may occasionally pursue non-dungeon adventures when the desire strikes them. New players and characters can come in or be swapped in at any point while the party is in town. No individual player needs to attend every session because no individual character needs to be on any given delve.

I, the DM, promise to prepare for that. The players promise to bring characters that will eagerly pursue that cadence of play. It doesn't create any unspoken requirements to attend every session for the sake of keeping in the loop of the plot. It doesn't mislead players as to what they will be doing. And most importantly, it takes a hard line on the boundaries of the play area, but within that play area, there is real choice with real consequence. No Quantum Ogre, no DM fiat.

And so, if I designed a ruleset around that contract, and DMed the game around that contract, I would be able to actually improve the game from session to session because I and the players would be pursuing the same goal. Which finally brings me to Delvers of Dàrkmesa.

Once I landed on the idea of the megadungeon as a distillation of the time-tested campaign promise, I thought about making my own take on Dwimmermount (if only for the supremely euphonic nature of the name), but decided instead to go completely homebrew. That's how Dàrkmesa came to be. I was struck by the double meaning of mesa (a huge rock formation that could easily fit a megadungeon inside, as well as the Spanish/Latin root for table), and what would a dungeon be without darkness? The Delvers part is just to drive the point home. Both in the sense that the characters will be primarily engaged in exploring a dungeon, and in the sense that the game is about them and about their exploration, not some arbitrary story I have concocted.

The rules and classes and example spells to include all fell into place now that I had a gameplay goal in mind other than "have fun." Many of the strangest parts of old school games that I could never understand before started to make sense and became incredibly attractive aspects of the experience for me.

Of course the game should be deadly, because that is a large part of what creates drama in a campaign with no forced plot. Of course random encounters were necessary, to keep the players and the DM honest as they all pursued the campaign goal: random chance is what keeps a non-narrative game alive. Of course two hundred pig-faced orcs appearing in a dungeon cavern is a good encounter, because it forces the players to bring to the table what makes them good players, not well-statted characters. And finally, of course the dungeon is the primordial soup from which all good games emerge, because it is the most apt compromise between what the DM needs to do, and what the players need to do. It creates strict boundaries that allow the DM to prepare more of the right thing, but when done right it forces no specific action on the players other than to engage. They want the treasure, and however they get it is by definition a fine way to play. That's why we all play this game, to get the treasure that exists hidden in the forgotten halls of our collective imagination and good company. So why hide the ball? Why not be clear about how agency works in the game right from the beginning? Why not embrace the canvas that was originally conceived for the game, and was so important that it became the first word in its name?

Luckily, it only took me ten years of searching to figure that out. Now I have the rest of my time to play.

Saturday, October 20, 2018

Gehenna & Harrowheim - Session 3

Before the party leaves Hammersted with the shipment of liquors, Grant distributes some "leftovers" along with a very terse mention of how he appreciates their help. The items he gives out are as follows:

  • Gaudith's Gauge: An intricately carved wooden spoon which, when set afloat in drinkable water, will act like a compass toward the thing (purposefully vague) that the bearer focuses on. The idea is that the bearer must be personally familiar with the thing, and they must succeed on a DC 18 Insight check or the attempt fails and they cannot use the item for a long rest.
  • Ranulph's Runcible: An ivory spoon, so clear and smooth that it appears to be carved of pure pearl. When used to stir a poisonous or toxic beverage or soup, it becomes pitch black.
  • Long-Shadow Shield: This was a shield that belonged to one of the dwarves who was lost (read: went out like a bad-ass) when the goblins first attacked. It is made of ironwood, which is a famous Faewild crafting material that is stronger than steel but a third the weight. The wielder has an additional 60ft of darkvision at night (60 if they had none before, 120+ if they did).

The party eagerly accept the items and learn approximately of their abilities with some Arcana checks. Then they set off back to the Stone's Throw. Niobe's panther, Frida, and Derek (wild-shaped into a donkey) pull the cart. Derek is asked later why he didn't become a camel or some other desert-acclimated creature, and he just shrugs. It seems that the dwarf druid isn't actually that familiar with mammals and such.

Halfway back, as they approach the ruined tower where they had battled the automaton, they spot a massive sandstorm (think this) speeding toward them from the southwest. At the forward apex of the storm is a massive black bird shape that only a couple members of the party even spot.

The party decides to speed toward the tower, which turns into a series of Constitution checks to maintain the full sprint. Mykos casts fly on himself and is able to carry Navo and Niobe to the cover of the tower just before the sandstorm hits.

Derek, Frida, and Calphire are all still on the cart a couple dozen yards from the tower when it is up-ended by the winds and sand, burying them. Navo and Mykos succeed in taking cover behind the tower wall and avoid the brunt of the damage. Niobe looks back out of fear for Frida and her hesitation plays out well as she fails her saving throw and is thrown away from the wall by the storm.

During the storm, Navo and Mykos spot stone men walking amidst the sands like zombies. They can't make out any specific details in the flurry.

After a few minutes, the storm passes to the northeast and the stone men have left no traces of themselves. Mykos and Navo begin frantically trying to dig out their companions, which was aided by the use of Gaudith's Gauge. Other than some bludgeoning damage (and a failed attempt to burn oneself free with a firebolt) everyone made it out unscathed.

One of the cart's wheels, however, was wrecked. Luckily, none of the casks were compromised. The party stashed the casks in the ruined tower under some sand, made their way back to the Stone's Throw, explained what happened, showed proof that they had completed the job and that the casks were awaiting pick-up, collected their 2000 gold pieces, and went to bed.

Thursday, October 4, 2018

Gehenna & Harrowheim - Session 2

Session 2 begins right outside the open gate of the Hammersted brewery, where the party peers in to see an armless corpse smeared with shit hanging behind the doors by a foot. Niobi's panther, Frida, begins to get wild eyes at the smell of goblins (and goblin excrement) in the air, and the party decides to kick in the door and take the fort by force.

A battle ensues where the majority of the party holds off several waves of goblins using the gate as a choke point, while a couple other PCs sneak around the back and discover a group of dwarves locked in the privy on the second floor of the main distilling building.

The dwarves are certainly happy to be free (and drinking) again, but their boss, Grant, is a rather prickly personality, and the party decide they'd rather sleep off their wounds and take a shipment of liquor back to the Stone's Throw than work out any further deal, especially after grant requests that the party train some of his new recruits in the martial arts...but doesn't offer any payment in exchange.

The party go to sleep, and that's where we end it for the night.

In retrospect, the battle took pretty much exactly as long as I had hoped, but the way that the party used the gate as a choke caught me off guard. The goblins were obviously not meant to be a tactical force, but I had designed the encounter and the goblin's layout within the brewery to encourage a stealth mission. The way I had envisioned it playing out was that they would take out 4-8 goblins without alerting any, and then some failed stealth or goblins that survive to take an action would alert the rest. Then, the party would have to take a defensive position while 12-16 other goblins charged their position.

Also in retrospect, I didn't like how little loot I had prepped for these goblins. I know there is a $2k reward once they get back to the tavern, but the end of the battle fell a little flat compared to the cool gem they were able to take from the golem last session.

I also wonder about Grant, the dwarf NPC and how "unlikeable" I made him. I definitely have his personality down, but the PCs didn't seem to want to engage him at all. His other dwarf employees: Talcum (the young, shy one), Mondr (the big silent type), Skol (the raspy-voiced intelligent one), and the new twins for security Herod and Kivki, didn't really get any screen time.

The plan is to have Grant slip the party some goodies before they leave to sweeten the deal and try to get on their good side a little. Also, the consequences of not training Herod and Kivki will probably come up a while later.

Wednesday, September 26, 2018

Gehenna & Harrowheim - Session 1

Session 1 begins with the last PC, Mykos, walking into the tavern after they have closed it up to keep in the warmth during the desert night. Mykos is a tiefling hexblade. He begins chatting with the bartender when he's approached by a human man in faded purple scale mail.

The man says his name is Bertrand, and he is a Dog of Yaangugar, the elite "police" of the sorcerer king Yaangugar, who rules a city closer to the easter coast of a continent. This city, Tannjerou, is where this man is interested in taking some well-equipped and capable adventurers to investigate the murder of a famous thespian ("Beorhtio"), allegedly by a radical group called the Black Flame Zealots. Finding the killers without raising suspicions from the group would guarantee a lot of money as a reward, in addition to Yaangugar's favor.

The PCs eventually all sync up and decide to help out Grouse with his lack of liquor first, and then meet Bertrand back at the Stone's Throw tavern to make their way east.

Before everyone goes to sleep, Navo's player reveals in a solo moment that his character's right arm has been badly crippled, and he is learning to throw daggers again with his left arm instead.

The party sets out for Hammersted along "Old Mill Road" and comes across what is presumably the ruins of the old mill. However, Derek the dwarf immediately recognizes that this is likely not a windmill, but the top of some corner tower of a giant castle or palace that is buried and very old.

The party goes to investigate when a giant scorpion comes around the bend. Before the scorpion can close with the party, a pile of what looked like sandstone erupts from the ground and takes the shape of a giant earth golem with clockwork mechanisms holding it together.

A battle ensues in which the golem throws the scorpion corpse at the party, almost killing Navo (who has very very very few maximum hit points somehow). Mykos ends up doing some great damage to the construct thanks to hexblade's curse, and Derek hits it with a lightning bolt. Notably, the creature is able to "slow time" before it is hit by a projectile using some gemstone in its hands, which Calphire pries out after the beast is defeated.

The party notes this location as a potential place of adventure in the future, but carries on into the desert. After a few more leagues they find the brewery. They begin discussing strategy for proving they are here on official business, but when they round the corner, they see the gate ajar.

And that is where we left it for the night.

Gehenna & Harrowheim - Session 0

This series is going to be a very simple log of a 5e homebrew campaign that I am now running on Fridays. I don't have much time to write these, so they will be summaries and highlights.

This is the setting pitch for the campaign:

The world has no name, but the destruction of the world has a name, and it is Gehenna. This is what the people call the ever spreading desert at the center of the large continent. The marching sands bring with them demons and dark magics that have minds of their own. As civilization has retreated, treasures, gold, and magics from another age were lost or abandoned, buried in now sunken ruins and caverns that await visits by curious adventurers.

Harrowheim is an ancient stronghold in eastern Gehenna. Where it stands was once a wooded cliff, lorded over by a vampire that had sewn powerful wards into the walls and grounds. Now, a hostile desert surrounds the oasis of dark wood and stone, but fails to penetrate the old dweomers of the place. As Gehenna has pushed past the fortress, the unfinished road from Harrowheim to the coast has gradually fallen into disrepair. What once was a guarded thoroughfare is becoming a highway of terrors, and frequent monster attacks, particularly at night, keep travelers at bay. Harrowheim houses a syndicate of worldly elves (drow) of varied aims, and abbutting the fortress is a shantytown of restless refugees. The political situation is... tenuous, to say the least.

The coast to the east, the Fertile Coast, hangs on to the luxuries of the past. Things are much the same as they were years ago, before the desert began to engulf the world. But every place on the continent dances on the edge of a knife, and on the coast that knife is fought over by two powers.

One force descending upon the coast are the waves of refugees fleeing from the desert. What might otherwise be just a large nuisance to local lords has marshalled into an insurgency. A group calling themselves the Black Flame Zealots has emerged from the refugees and recruited starving young men and women into their ranks. Wearing the image of the infamous black phoenix, the zealots have assassinated prominent figures on the Fertile Coast, burned down government buildings and stoked revolutionary ideas in the major cities.

Sorcerer Kings are rising among civilized lands that have become desperate to keep the Zealots at bay. These beings run the gamut of morality, but they are all willing to crush rebellion and treason with an iron fist and burn the will of the insurgents to ash with arcane fire.

Meanwhile, the old gods are being abandoned daily as the world becomes a husk, and a new faith, the Burning Erudition, is taking root among refugees and settled peoples alike. The cults of this new religion are varied and fractured, but prominent figures are beginning to emerge and seize power.

Adventurers who stray deep into the woods and swamps of the world may find themselves stumbling into the Fae, a place and a peoples of singular purpose: to toy with outsiders and belittle their mortal machinations. The faces of the Fae can range from benevolent elves to the murderous Wild Hunt.

Stretching vast around the other planes of existence is the Astral Sea, a mythical starfield that houses whole continents suspended in its aether. Only great civilizations have achieved the magic necessary to send their peoples into the astral plane. Some, such as the high elves (eladrin), used it as a new home. Others, like the giants, use it as a cosmic prison for the worst of their kind.

And we haven’t even mentioned the Spirit World or the Underdark...
Session 0

After character creation, we begin in a tavern called The Stone's Throw, a few leagues out from the edge of the spreading desert. It is run by a human man names Grouse, and his half-drow half-human daughter Minna.

Already in the tavern were two player characters: Calphire, a halfling wild sorcerer with hair like blown glass, and somewhere else in the tavern Niobi, a half-elf ranger (homebrew version of the class that I'll post some time) and her panther companion Frida.

The first of the players to enter the tavern after play had started was Derek, a dwarf druid. He came in, asked for a glass, cast create water into it, and started to drink. The bartender asked him for some money for renting the glass, and Derek promptly threw the water into Grouse's face and walks out. He runs into Hob, the Goblin merchant sleeping in a barrel outside, but doesn't buy any of his magic wares. He goes to sleep in the stables.

Next in was Navo, a half-elf arcane archer who uses daggers instead of a bow and arrow. He tried chatting up the ranger and her panther pet, but was unsuccessful.

The last PC is delayed because his player can't make session 0.

Calphire goes over to the edge of the bar where various jobs are posted and finds one put there by the barkeep himself. His shipments of rare desert liquors from the Hammersted brewery stopped a month ago and he needs someone to go check out why. The brewery is further into the actual desert a few leagues. He's promising a fair amount of money. Calphire accepts, then goes to the two half-elves to talk teaming up for the job.

I end the session there, because we're already well past three hours due to character creation.

Tuesday, November 7, 2017

5th Edition Dungeons & Dragons: The Action Economy "Problem" Phantom

Blah blah, haven't posted in forever because of real life stuff, etc. etc.

I recently started following a bunch of D&D subreddits (because I'm clearly an old man in this digital age), and a "problem" with 5th edition that I have seen discussed on multiple occasions is the action economy.

The first place I heard it spoken of was on Matt Colville's Youtube channel, in this hour long video, I think.

Colville didn't say it was a "problem" so much as he simply made an observation: 5th Edition D&D math stacks the odds in the PC's favor when it comes to single-opponent boss fights, especially later on in the levels.

Essentially, what makes the difference in 5E combat is how many opportunities each side has to act. Single, powerful monsters have a comparatively limited number of actions to do each round when fighting a party of PCs. Even with the "legendary action/resistance" powers available to the truly dangerous enemies, their ability to capitalize on any advantages in the flow of combat are cut short by the sheer number of PCs that get turns in between each true action the enemy can do.

So let's say you, the DM controlling a dragon, really smash one of the characters with a critical hit. That character is now close to dead...

In the early levels of the game, one of the PCs would have needed to spend their entire turn healing the characters to make sure they survived. This means the dragon is effectively facing one fewer enemies this round. Do that every round and the PCs have a dire situation on their hands.

But in the later levels of the game, the party can save that PC using only bonus actions (namely Healing Word). So even with the same level of damage output, the monster in this circumstance has a much lower chance of winning a straight fight because it is (effectively) fighting an extra PC every round.

The legendary actions allow the DM, in this case, to have the dragon do a tail swipe or some such attack out of turn to try and further complicate that PC's life and, hopefully, take a PC out of the fight temporarily to relieve some of the pressure.

But there are other ways to slow down the PC's action economy. Here are a bunch, briefly listed:

  • A flying creature battles the PCs on a crumbling mountain side. Random PCs must Dex save each round or slide down 10ft. The monster regularly tries buffetting the PCs off balance to inflict falling damage.
  • A hyper intelligent enemy has an ever-shifting rubiks-cube-esque boss chamber that it can command to move with a thought. Every round the PCs are slid into walls or each other, out of formation and into spell areas, etc.
  • The BBEG runs away, with the intent of being chased, to lure the party into a secondary trap/encounter/difficult terrain, etc.
  • The bad guy is rather alchemically inclined, and throws tanglefoot bags and alchemists fire and acid vials as bonus actions.
  • The BBEG has a dancing weapon that they reveal in the second round of the fight. Now there are two sources of attacks each round.
  • The evil wizard casts polymorph on his familiar in the first round of combat and makes it into a crazy monster.
  • The fight occurs in a room full of portals that interconnect. Run into one and you pop out of another. Only the BBEG knows which one leads where.
  • Your smaller evil guys can stealth! If they hide behind a tree, they can move away and attack again from a better vantage point.
  • Make use of semi-permanent choke points. The bad guy leads the party into a tight alleyway, but faster and athletic characters can take a different side street or climb up a building's siding. Either way, it distracts certain PCs for a round or two.
  • The fight occurs in the swamp, which is waist-high water (difficult terrain for medium creatures, swimming only for small ones) that doesn't slow down the BBEG cuz he lives here.
  • Start the fight with the PCs in a hole or down a level or two from the bad guy. Let the BBEG shoot magic or arrows or bombs down at them as they climb for the first couple rounds.
  • Big enemies grab PCs. They grab them all the time and throw them into other PCs. They use PCs like makeshift clubs, or toss them into spiked pits or out windows, etc.
  • Make the point of the fight to stop a ritual or destroy an item, so that if the PCs focus completely on dealing damage to the monster they will fail the main goal.
You get the drift. Change the playing field every round. Keep the PC's on their toes. The monster rules will not give you everything the villain should have at their disposal. If your PCs are using crazy ideas and tactics, your villains should too.

And ultimately, the easiest way to make sure that your PCs don't trounce a big baddie because they get waaaaaay more actions each round is to add more enemies. Evil duos are awesome. Evil leaders (bard, clerics, etc.) with a small battalion of mooks are awesome. A wizard with a golem for protection and a pet chimera makes sense, and makes for crazy combat.

The books have the rules in them, but the real challenges you present to your players can't be found in the books. The challenge is in the way you design the fight and force your players to shift gears multiple times before it's over.