by Mike Shea on 29 June 2015
Even with the fast combat of the 5th Edition of Dungeons and Dragons, we might still need some tricks up our sleeves for running large piles of monsters against a number of PCs. Luckily, 5e D&D has already given us an idea about how these swarms of monsters might work by giving us lower level critter swarms such as the Swarms of Bats, Swarms of Rats, and Swarms of Quippers (yeah, I had to google that one too).
But what if we want swarms of goblins, swarms of zombies, or swarms of ghouls? What if we want to challenge our high level PCs with dozens of monsters? Yes, the flat math of 5e makes that possible but who wants to roll fifty attack rolls? What if, instead, we treat swarms of skeletons the same way we treat swarms of bats?
Say our 12th level band of adventurers gets attacked by swarms of bullywugs. Instead of actually counting out dozens of bullywugs, we can take the stat block for a hill giant as our base statistics for a swarm of ten bullywugs. Four hill giants would turn into four swarms of ten bullywugs each. All of the attacks and defenses would stay the same. As far as the player knows, the two attacks of a hill giant were actually the aggregate of a number of different attacks from the bullywugs. The size of the swarm is actually huge instead of the large size of a hill giant to accomidate all of the bullywugs.
This swarm of bullywugs likewise gains the effects of other typical swarms. Here's a list of the swarm traits:
Smaller swarms also have one additional trait:
You might decide to not add this swarm effect since medium-sized creatures in a swarm would be just as suseptable to that as they would individually.
How many monsters are actually in a swarm? You can figure that out by dividing the hit points of the creature you reskinned by the number of hit points in the original monster. If a normal bullywug has 11 hit points and a hill giant has 105, that's about ten bullywugs in a single swarm (105 divided by 11).
You can create larger or smaller swarms by picking higher or lower CR monsters to reskin. When reskinning a monster into a swarm, pick as generic a monster as you can find. Instead of using the effects of the base monster, use the effects that the smaller monster would use. Giants, in particular, are great monsters to reskin into swarms because they're so straight forward in their mechanics.
It works best if you use two or three monsters rather than just one big swarm. Three swarms of ten bullywugs (actually three reskinned hill giants) is better than one huge storm-giant-sized swarm.
There are a lot of things players what to do with high-level swarms. What if a clerics casts turn undead on a swarm of skeletons? In general, single target spells won't have much of an effect on a swarm. Things like Turn Undead are best ruled on as they happen. What about a fireball? Shouldn't it do a lot more damage against a horde of bullywugs? You could rule they are vulnerable if you want but generally speaking you can describe how some of the bullywugs got the brunt force of the attack while the rest managed to escape. The full force of the fireball will still take off a bunch of hit points.
In general, go with whatever sounds cool.
There's another technique for running tons of lower CR monsters that goes back many years and is even codified in the 13th Age game system. Rather than track separate hit points for a pile of monsters, you pool all their hit points together into a big pool. Ten bullywugs would have 110 hit points in a pool. Every 11 damage, another bullywug is killed.
When they attack, they would still attack with a bunch of d20 attack rolls but you could do them with a handful of dice and just use the average damage in the Monster Manual if they hit. If these monsters get hit with an area attack, apply the full damage to the pool and count off that many kills.
For some groups this might be a lot more realistic than a horde of bullywugs that actually feel like a hill giant.
However you run them, these techniques for running swarms of monsters are great ways to get that feeling of many-on-few battles without spending a ton of time on bookkeeping. Give them a try, see if they work, and, if they do, use it as another tool for running fun and fantastic 5e battles.