n large amphibious animal swarm 

Perception +9; tremorsense 30 feet

Skills Athletics +8, Stealth +11

Str +0 Dex +3 Con +4 Int -5 Wis +1 Cha -5

AC  19; Fort +12, Ref +11, Will +9

HP 40; Immunities precision, swarm mind; Weaknesses area damage 5, salt 5, splash damage 5

Speed 5 feet, swim 20 feet

Blood Draining Bites [>] Each enemy in the swarm's space takes 2d6 bleed damage and is exposed to brood leech swarm venom.

Brood Leech Swarm Venom (poison) Saving Throw DC 21 Fortitude; Maximum Duration 6 rounds; Stage 1 clumsy 1, sickened 1, and –5-foot status penalty to Speed (1 round); Stage 2 clumsy 1, sickened 1, and –10-foot status penalty to Speed (1 round)

Bestiary 2, page 156.