Blood Feast◆ Each enemy in the bat swarm's space takes 1d4 piercing damage (DC 16 basic Reflex save). Creatures that fail this save also take 1 bleed damage.
Source: Pathfinder Monster Core
Running this fight? Add the vampire bat swarm to the initiative tracker: type its name and the AC, hit points and Perception for initiative fill in, and the book button shows this stat block mid-fight.