Summon Swarm

The swarm has the advanced simple template. A swarm of rats or spiders leaves behind a trail of filth or ichor when it moves, which has the effects of a grease spell for 1 round after the swarm passes through an area. Any creature that moves through the affected area is also subject to the swarm's disease or poison, as if attacked by the swarm. A prone creature in the affected area takes a –4 penalty on its saving throw against this affliction. A bat swarm leaves behind a similar trail that lasts for 2 rounds but doesn't have a disease or poison effect.