Leech Swarm
A mass of wriggling black creatures each the size of a man's finger stirs the stagnant water before enveloping its victim.

Leech Swarm (CR 4; XP 1,200)
N Diminutive vermin (aquatic, swarm)
Init +4; Senses blindsight 30 ft.; Perception +0
Defense
AC 18, touch 18, flat-footed 14
(+4 Dex, +4 size)
hp 39 (6d8+12)
Fort +7, Ref +6, Will +2
Immune mind-affecting effects, swarm traits, weapon damage
Weaknesses susceptible to salt
Offense
Speed 5 ft., swim 30 ft.
Melee swarm (2d6 plus poison)
Space 10 ft.; Reach 0 ft.
Special Attacks blood drain, distraction (F-DC 15)
Statistics
Str 1, Dex 18, Con 15, Int —, Wis 10, Cha 2
Base Atk +4; CMB —; CMD
Skills Stealth +16 (+24 in swamps), Swim +12; Racial Modifiers +8 Stealth in swamps, uses Dexterity to modify Swim checks
Ecology
Environment marsh
Organization solitary, pair, or infestation (3–6 swarms)
Treasure none
Special Abilities
Blood Drain (Ex) Any living creature that begins its turn with a leech swarm in its space is drained of its blood and takes 1d3 points of Str and Con damage.
Poison (Ex) Swarm—injury; save Fort DC 15; frequency 1/round for 2 rounds; effect 1d4 Dexterity drain; cure 1 save.
Susceptible to Salt (Ex) A handful of salt burns a leech swarm as if it were a flask of acid, causing 1d6 points of damage per use.

This horrifying cloud of ravenous, blood-draining parasites eschews the stealth of a lone leech's methods in favor of swift and merciless feeding.