From publisher blurb:
A Bigger, Better Monster is a PDF series helping gamemasters improve their fantasy gaming monsters. It is system neutral, but covers all the basics of what the original monsters appears to do, and how to customize them for your own adventures and campaigns.
This PDF shows how to turn swarms from just another encounter into a different kind of encounter. In this system, swarms do not have armor class, or defenses, or initiative. Instead, they are thoroughly vexxing obstacles that are neither combat, nor traps.
From Page 2:
There are two kinds of swarms: animals and magical things. Animal swarms are bats, bees, rats, snakes, spiders, wasps, and so on. They are annoying and only really matter at low-levels. They have a bevy of immunities — you can’t stab an insect swarm — and most PCs hate them.
Magical swarms are less frequent, and as I’m writing this I’m wondering if anyone has ever even written one. But I’m going to cover it as well.