Clinging Bites◆ The trilobites in the swarm latch onto creatures and gnaw at them. Each enemy in the swarm's space takes 2d6 slashing damage (DC 18 basic Reflex save).
Source: Pathfinder Monster Core 2
Running this fight? Add the trilobite 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.