Swarm, Cranium Rat Average Pack (CR 5)

Medium Magical Beast (Extraplanar and Swarm-Diminutive)
Alignment: Usually neutral evil
Initiative: +7 (+3 Dex, +4 Improved Initiative); Senses: darkvision 60 ft., low-light vision, Listen +19, and Spot +19
Languages: telepathy 80 ft. to any creature with language

AC: 14 (+3 Dex, +1 natural), touch 13, flat-footed 11
Hit Dice: 12d10+24 (90 hp)
Fort +10, Ref +11, Will +8
Speed: 40 ft., climb 20 ft.
Space: 10 ft./0 ft.
Base Attack +12; Grapple -
Attack: Swarm
Full Attack: Swarm
Damage: Swarm 3d6
Special Attacks/Actions: Distraction (DC 18), mind blast, spells
Abilities: Str 2, Dex 17, Con 14, Int 13, Wis 14, Cha 13
Special Qualities: Cold resistance 10, hive mind, merge swarms, swarm regeneration, swarm traits
Feats: Alertness; Combat Casting; Improved Initiative; Iron Will; Spell Penetration
Skills: Climb +19, Listen +19, and Spot +19
Advancement: -
Climate/Terrain: Any land and underground (any Outer Plane)
Organization: Solitary, pack (2-4 swarms), or infestation (7-12 swarms)
Treasure/Possessions: -

Source: Fiend Folio

Mind Blast (Su): This attack is a 60-foot cone. Anyone caught in this cone must succeed on a Will save (DC 17) or be stunned for 3d4 rounds. An average pack can use this ability at will.

Spells: An average pack can cast arcane spells as a 4th-level sorcerer (spells/day 6/7/3; spells known 6/3/1; save DC 11 + spell level). A typical spells known list: 0 - daze, detect magic, flare, mage hand, open/close, prestidigitation; 1st - charm person, expeditious retreat, magic missile; 2nd - minor image.

Hive Mind (Ex): A cranium rat swarm has a hive mind, which makes it susceptible to mind-affecting spells. For purposes of such spells, the swarm is a single creature of the magical beast type.

Merge Swarms (Ex): Two lesser cranium rat packs can move into the same space and merge into a single swarm, becoming an average pack. Likewise, two average packs can merge together into a greater pack. Merging in this fashion is a full-round action.

Swarm Regeneration (Ex): When an average pack is reduced to 0 hp, it becomes a lesser pack. When a lesser pack is reduced to 0 hp, it is destroyed. This transformation is instantaneous.

Telepathy (Su): An average pack of cranium rats can communicate telepathically with any creature within 80 feet that has a language.

Skills: Cranium rat swarms have a +8 racial bonus on Climb checks and can always choose to take 10 on Climb checks, even if rushed or threatened.

Distraction (Ex): Any non-mindless creature vulnerable to a swarm's damage that begins its turn with a swarm in its square is nauseated for 1 round; a Fortitude save )DC 10 + 1/2 swarm's HD + swarm's Con modifier) negates the effect. Spellcasting or concentrating on spells within the area of a swarm requires a Concentration check (DC 20 + spell level). Using skills requiring patience and concentration requires a Concentration check )DC 20).

Swarm Traits: A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits or flanking. A swarm made up of Tiny creatures takes half damage from slashing and piercing weapons. A swarm composed of Fine or Diminutive creatures is immune to all weapon damage.

Reducing a swarm to 0 hit points or fewer causes the swarm to break up, though damage taken until that point does not degrade its ability to attack or resist attack. Swarms are never staggered or reduced to a dying state by damage. Also, they cannot be tripped, grappled, or bull rushed, and they cannot grapple another.

A swarm is immune to any spell or effect that targets a specific number of creatures (including single-target spells such as disintegrate), with the exception of mind-affecting effects if the swarm has an Intelligence score and a hive mind. A swarm takes a -10 penalty on saving throws against spells or effects that affect an area, such as many evocation spells or grenadelike weapons. If the area attack does not allow a saving throw, a swarm takes double damage instead,

Swarms made up of Diminutive or Fine creatures are susceptible to high winds such as that created by a gust of wind spell. To determine the effect of wind on a swarm, treat the swarm as a creature of the same size as its constituent creatures. For example, a swarm of Abyss ants (Diminutive creatures) can be blown away by a severe wind. Wind effects deal 1d6 points of subdual damage to the swarm per spell level or Hit Die of the originating creature, in the case of effects such as an air elemental's whirlwind). A swarm that takes subdual damage sufficient to exceed its current hit points becomes disorganized and dispersed, and does not re-form until its current hit points exceed its subdual damage.

While dangerous and unpleasant, cranium rats are not aggressive creatures. They avoid open attacks in favor of flight or ambushes. Cranium rats use their spells and mind blast ability to soften or incapacitate victims before swarming over them, then they drain their victims' blood through a hundred tiny wounds.

Extraplanar Subtype

A subtype applied to any creature when it is on a plane other than its native plane. A creature that travels the planes can gain or lose this subtype as it goes from plane to plane. This book assumes that encounters with creatures take place on the Material Plane, and every creature whose native plane is not the Material Plane has the extraplanar subtype (but would not have when on its home plane). An extraplanar creatures usually has a home plane mentioned in its description. These home planes are taken from the Great Wheel cosmology of the D&D game (see Chapter 5 of the Dungeon Master's Guide). If your campaign uses a different cosmology, you will need to assign different home planes to extraplanar creatures.

Creatures not labeled as extraplanar are natives of the Material Plane, and they gain the extraplanar subtype if they leave the Material Plane. No creature has the extraplanar subtype when it is on a transitive plane; the transitive planes in the D&D cosmology are the Astral Plane, the Ethereal Plane, and the Plane of Shadow.