Bats often congregate in large colonies in underground areas. Though bats are mostly harmless and bat colonies generally have little interest in adventurers or humans in general, if they are disturbed, they can prove dangerous to inexperienced adventurers.

Any Small or larger creature moving within 30 feet of a bat colony must succeed at a DC 15 Stealth check each round or use wild empathy or a similar ability to keep the bats calm, or else there is a 50% chance the bat colony becomes disturbed. Additionally, each round the bat colony is exposed to normal or brighter light conditions or loud sudden noises, there is a 30% chance the bat colony becomes disturbed.

If the bat colony is disturbed, the bats begin to fly about, shrieking and milling in an oversized swarm. When disturbed, the colony takes up a 20-foot-radius area. A creature that ends its turn in the area takes 1d6 points of damage, and is affected by the distraction and wounding special abilities of a bat swarm.

The colony remains disturbed for 1d4+1 rounds (or until the swarm is dispersed by damage), after which the bats either return to their previous position or flee the area, depending on whether they feel a threat is still present.