Bone Rat Swarm (CR 3)

Tiny Undead (Swarm)
Alignment: Always neutral evil
Initiative: +3 (Dex); Senses: darkvision 60 ft.

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AC: 15 (+2 size, +3 Dex), touch 15, flat-footed 12
Hit Dice: 4d12 (26 hp); DR: DR 5/bludgeoning
Fort +1, Ref +4, Will +4

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Speed: 15 ft., climb 15 ft.
Space: 10 ft./0 ft.
Base Attack +2; Grapple -
Attack: Swarm
Full Attack: Swarm
Damage: Swarm 1d6
Special Attacks: Distraction

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Abilities: Str 2, Dex 17, Con -, Int -, Wis 10, Cha 1
Special Qualities: half damage from slashing and piercing weapons, immunity to cold, swarm traits, undead traits
Feats: -
Skills: -
Advancement: None

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Climate/Terrain: Any
Organization: Solitary, pack (2-4 swarms), or infestation (7-12 swarms)
Treasure/Possessions: None

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Source: Libris Mortis

With the scrabble of hundreds of bony claws, a mass of tiny skeletal creatures surges across the floor, with pinpoints of red light gleaming in their empty eye sockets.

A bone rat swarm is a mass of undead skeletal rats. Though individually such creatures would pose little risk, in great numbers they can strip a creature clean in short order.

Combat

A bone rat swarm seeks to engulf and devour any living prey it encounters. A bone rat swarm deals 1d6 points of damage to any creature whose space it occupies at the end of its move.

Distraction (Ex): Any living creature that begins its turn with a bone rat swarm in its space must succeed on a DC 15 Fortitude save or be nauseated for 1 round. The save DC is Constitution-based.