Centipede Swarm
CREATURE 3Swarms of centipedes are dangerous indeed, ravenous carpets of skittering hunger capable of devouring a traveler whole in a matter of minutes. Kobolds and mitflits are both known to incorporate swarms of centipedes into cunning traps.
Hunters and scavengers that live amid dung and detritus, centipedes are a relatively common and often reviled threat faced by adventurers. Scurrying about with surprising speed on the scores of legs attached to their long, segmented bodies, centipedes strike with poisoned mandibles to slow and torment their prey with a vicious toxin before they settle down to feed in slow and disgusting leisure.
Tremorsense allows a monster to feel the vibrations through a solid surface caused by movement. It is an imprecise sense with a limited range (listed in the ability). Tremorsense functions only if the monster is on the same surface as the subject, and only if the subject is moving along (or burrowing through) the surface.
This monster doesn't have a single mind (typically because it's a swarm of smaller creatures), and is immune to mental effects that target only a specific number of creatures. It is still subject to mental effects that affect all creatures in an area.
Each enemy in the swarm's space takes 1d8 piercing damage (DC 20 basic Reflex save) plus centipede swarm venom.