n large animal swarm 

Perception +12; low-light vision, scent (imprecise) 60 feet

Skills Acrobatics +13, Stealth +11

Str +1 Dex +5 Con +3 Int -4 Wis +2 Cha -3

AC  18; Fort +11, Ref +13, Will +10

HP 50; Immunities precision, swarm mind; Weaknesses area damage 5, splash damage 5

Speed 30 feet, climb 30 feet, swim 30 feet

Venom Spritz [>>] The vipers spray venom from their fangs in a defensive display. Each creature in a 10-foot cone is exposed to viper swarm venom but gains a +2 circumstance bonus to its initial saving throw against the poison.

Venomous Fangs [>] Each enemy in the swarm's space takes 2d8 piercing damage (DC 21 basic Reflex save) plus viper swarm venom.

Viper Swarm Venom (poison) Saving Throw DC 21 Fortitude; Maximum Duration 6 rounds; Stage 1 1d4 poison damage (1 round); Stage 2 1d6 poison damage and clumsy 1 (1 round); Stage 3 2d4 poison damage, clumsy 2, and enfeebled 1 (1 round)

Bestiary 3, page 249.