Goodman Games

I have been thinking of a standard way to handle swarms of monsters and I came up with this basic template. I see this as covering all swarm-like monsters, including carnivorous bats, rats, insects, colonies of mold, slime, vines, pirhannas etc.

The swarm would act a single monster that occupies an entire room (and maybe adjacent hallways). For example, consider a small swarm of flesh-eating rats:

Hit Dice 5d12 (judge rolls 34). AC 15 against piercing attacks (reduce damage to 1 pt). Hit automatically by other attacks. Cannot be critically hit.

At 90% hit points or more (30-34), swarm is still disinterested. One attack per character walking through the swarm at +4 for 1d4 damage.

At 70% hit points (24-29), swarm is frenzying. One attack per character, which hits automatically for 1d8 damage.

At 40% hit points (14-24), swarm is in full frenzy. One attack per character, which hits automatically for 1d12 damage. There is a 33% chance that 1-3 Vapor Rats will emerge from the swarm as separate monsters.

At 20% hit points (7-14), swarm is breaking. One attack per character, which hits automatically for 1d6 damage.

At fewer hit points, swarm is broken. One attack per character at +4 for 1d4 damage.

The damage and levels at which the swarm changes could be different for different swarms. Bees might never break until the end, for example, while pirhannas would go into a frenzy very quickly and slimes might take longer to build up their attacks (as they are sliced into more and more slimes). The special "frenzied" event could change as well (maybe bats would have a small chance to pass on a disease, while vines might grab at characters and mold might release toxic spores). My goal is to make each swarm present a different fight, but a common principle. I see swarms as naturally docile roadblocks that keep players from accessing certain rooms and hallways (as fighting them can turn ugly, fast).

You mean do it basically as in D&D 3.5?

Maybe? I've never played past AD&D. Is that how they do it in 3.5?

Swarms are just a monster type, immune to non-magic weapons for the most part. Threatened by fire. They take up X number of 5 foot squares and are basically dispersed instead of being killed. Basically, instead of statting up a single bug, you create a "monster" composed of a lot of bugs and then play out logic required for a monster composed of a bunch of bugs. You can move through a swarm, giving it an automatic hit against, for example. Requires a Will Save to get through without giving in to the "fear"/ickiness.

I even wrote a PDF once about adapting the swarm rules for dealing with mobs/street riots and swarms of other "big creatures" like a swarm of stirges. A stirge is pretty formidable against 1st level characters but against 9th level characters, not so bad. But if they acted like a swarm, then they would be scary. I suppose for epic play you could have a swarm of dragons.... (That would work better in 3.x/4.x where Epic play (stuff way beyond name level) isn't monty haul or anathema.)