n large animal swarm 

Perception +4; darkvision, web sense

Skills Acrobatics +5, Athletics +2, Stealth +5

Str -2 Dex +3 Con +0 Int -5 Wis +0 Cha -4

Web Sense The spider swarm has imprecise tremorsense to detect the vibrations of creatures touching its web.

AC  15; Fort +4, Ref +7, Will +2

HP 12; Immunities precision, swarm mind; Weaknesses area damage 5, splash damage 5

Speed 20 feet, climb 20 feet

Swarming Bites [>] Each enemy in the spider swarm's space takes 1d4 piercing damage with a DC 14 basic Reflex save. A creature that fails its save is exposed to spider swarm venom.

Spider Swarm Venom (poison) Saving Throw DC 14 Fortitude; Maximum Duration 4 rounds; Stage 1 1 poison and enfeebled 1 (1 round); Stage 2 1d4 poison and enfeebled 1. (1 round)

Bestiary, page 306.