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Everyone knows solo monster fights don’t work well in 5e given the action econ issues. I call 'em "solo" rather than "boss fights" because as DM, you'll have a better time if you don't think of your locations in terms of a forced ludonarrative arc instead of letting the players be in charge of where they go and where they focus their energy. Lost Mine of Phandelver: they one-shotted Nezznar but were in terror of the maze spiders picking them off one by one. (Then Venomfang killed them but that wasn't satisfying either.) Curse of Strahd: they died to 3d6 wolves over and over but actual Strahd was a cake walk.
But if a group of enemies works so well, why not use the symbolic mechanics of a group of monsters but run them diegetically as a whole monster?
Here’s a CR 1½ example: both arms are crocodiles (they can grab a PC, and if they are already holding a PC they can squeeze or slam or twist them), the mouth is a giant frog (great for eating them), and their sword&bow are represented by the stat block of a skeleton. Now, I don’t mean to say that this creature needs to look like something out of the worst nightmares of the He-Man design studio in the eighties. It can, if you want, but it can also just look like a great warrior. The hands look like hands, the weapons look like weapons, the face looks like a face; I just mean to take the stat blocks of that sundry menagerie and jam.
They all have their own separate initiatives i.e. they can act between the player characters. If one of parts move, so does the other parts. (So lower their speed so the overall speed across a turn won't be too high, unless you're doing theatre of the mind.) The hit point pools are separate, too. If the characters break one part then that part’s attacks can’t be used, and it will act more seldomly in the round too. Maybe it’ll go like this: they’ve broken the fingers of both hands (“crocodile” stat blocks are dead) and they’ve smashed the mouth (“toad” stat block is dead) so all it has left is chopping desperately with the sword (“skeleton” stat block). Or maybe one part will be charmed or one part will resist a fear spell (“She’s terrified, her face turning away in disgust, but she slashes with her shortsword at you, Alice.”) Or maybe the party will one-shot the entire concoction with a lightning bolt or something. Do not get attached to outcomes or to the dream of an "epic boss". Play to find out.
There can be so much variety in this setup; put a healer in the mix and it can patch itself up. Put charming sirens or fireball-hurling flameskulls in there.
I came up with this idea in 2014 and I’ve occasionally see others independently find the same thing, but I rarely see it mentioned as often as other, more cockamamie approaches to solving bossfights get proposed.
This is the opposite of a swarm (which is combining many creatures into one statblock / HP pool / action count); instead, this breaks up one creature into many statblocks / HP pools / action counts.
Speaking of swarms, they can be really unsatisfying. Here you are picturing a whole bunch of creepy crawlies but the over-abstract nature of the swarm rules in 5e makes them feel like a li’l weenie, barely an inconvenience. “Oh, it’s just a swarm.”
I tried to show in the “Solo monsters” section above how awesome it can be when a monster gets to act interleaved with the heroes. So swarms are frustrating to me because it makes them just as “non-epic” as a single creature.
I’ve tried rolling the swarms HP same way I would roll up how many wolves there are, and describing the players reducing its hitpoints as them killing individual bats, but they saw through that soon enough and were back to yawning at the swarms.
Thanks to mob rules and momentum attacks, it’s pretty easy to handle hordes of standalone enemies as easily as any swarm. Maybe it’s time to break up the swarms for real? To reverse-engineer the mob rules math to find out an appropriate amount of attacks and damage for the swarm.
Let’s see here… a swarm of centipedes, 5d8 HP so I wanna tie each bug to that, something like bugs having one HP each, or 2 HP each, or even ten bugs per HP can be explored. Let’s try one each for this example, so we have an average of 22 centipedes showing up. The entire swarm has one single attack in the entire round, and it’s a measly +3 to hit, for a not-too-shabby 4d4 if the whole swarm is alive or 2d4 if it’s half gone.
So that damage output is a good benchmark. We can have each bug be able to bite for 1d4, but lower the chance to hit to about a fifth so the average total damage doesn’t change too much. Looking at the mob rules table to see how much we need to go down from a 1 to a 5, eleven steps, so that means going from +3 to hit down to a -8 instead. Double-checking that, Monte Carlo style, against a range of armor classes says we’re in the right ballpark, with the “individual approach” being a tiny bit weaker against AC 12 and 13, a tiny bit stronger against any other AC, but low deltas all around.
So that swarm of is now 5d8 individual bugs all that hit at -8, or attack value 2, for 1d4 each, for the occasional scary beat but still managable with momentum attacks from the players and usable with mob rules: count them in units of five where each five counts as one +3 attacker—you’d need several dozens of them to be a threat, but that might happen in case the party runs into many, many bugs at once. The swarm is half a CR, so at deeper dungeon levels maybe they’ll run into 20d8 centipedes (the equivalent of four swarms).
This is the sort of math hack that would be much harder in a dice curve system, and it’s much easier with the linear d20 we have in D&D.