n large animal swarm 

Perception +8; low-light vision, scent (imprecise) 30 feet

Skills Acrobatics +10, Stealth +8

Str +0 Dex +4 Con +2 Int -4 Wis +2 Cha -3

AC  16; Fort +8, Ref +10, Will +6

HP 20; Immunities precision, swarm mind; Weaknesses area damage 3, splash damage 3; Resistances bludgeoning 3, piercing 5, slashing 5

Mass Wriggle [R] Trigger The rat snake swarm takes damage from a melee Strike; Effect Snakes slither up and around the creature's weapon and limbs. The target must succeed at a DC 15 Will save or become frightened 1.

Speed 20 feet, climb 20 feet, swim 20 feet

Swarming Strikes [>] Each enemy in the swarm's space takes 1d8 piercing damage (DC 17 basic Reflex save).

Bestiary 3, page 249.