General Rules: Monsters - Bone Rat Swarm

Bone Rat Swarm (adapted from Libris Mortis: The Book of Undead and Dragon Magazine #329)
Tiny Undead (Swarm)

Hit Dice: 4d12 (26 hit points)
Initiative: +3
Speed: 15 feet, climb 15 feet
Armor Class: 15 (+2 size, +3 Dexterity); touch 15, flat-footed 13
BAB/Grapple: +2/-
Attack: Swarm (1d6)

Full Attack: Swarm (1d6)

Space/Reach: 10 feet by 10 feet/0 feet
Special Attacks: Distraction

Special Qualities: DR 5/bludgeoning, darkvision 60 feet, half damage from slashing and piercing weapons, immunity to cold, swarm traits, undead traits

Saves: Fortitude +1, Reflex +4, Will +4
Abilities: Str 2, Int -, Wis 10, Dex 17, Con -, Cha 1
Skills: -
Feats: -
Environment: Any
Organization: Solitary, pack (2 - 4 swarms), or infestation (7 - 12 swarms)
Challenge Rating: 3
Treasure: None
Alignment: Always neutral evil
Advancement: None
Level Adjustment: -

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

In order to attack, a bone rat swarm moves into opponents' spaces, which provokes attacks of opportunity. It can occupy the same space as a creature of any size, since it swarms all around its prey, but remains a creature with a 10 foot by 10 foot space. Bone rat swarms never make attacks of opportunity, but they can provoke attacks of opportunity. A bone rat swarm’s attacks are nonmagical. Damage reduction sufficient to reduce the swarm attack’s damage to 0, being incorporeal, and other special abilities usually give a creature immunity (or at least resistance) to damage from a swarm.

Unlike other creatures with a 10 foot by 10 foot space, a bone rat swarm is shapeable. It can occupy any four contiguous space and it can squeeze through any space large enough to contain one of its component creatures.

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

Swarm: A swarm has no clear front or back and no discernible anatomy, so it is not subject to critical hits or flanking. A bone rat swarm is immune to all weapon damage. Reducing a swarm to 0 hit points or lower causes it 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 an opponent. 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 (charms, compulsions, phantasms, patterns, and morale effects) if the swarm has an Intelligence score and a hive mind. A swarm takes half again as much damage (+50%) from spells or effects that affect an area, such as splash weapons and many evocation spells. A bone rat swarm is susceptible to high winds such as that created by a gust of wind spell. For purposes of determining the effects of wind on a bone rat swarm, treat the swarm as a creature of Diminutive size. A swarm rendered unconscious by means of nonlethal damage becomes disorganized and dispersed, and does not reform until its hit points exceed its nonlethal damage.

Undead Traits: A bone rat swarm is immune to mind-influencing effects, poison, sleep effects, paralysis, stunning, disease, death effects, and any effect that requires a Fortitude save unless it also works on objects or is harmless. It is not subject to critical hits, nonlethal damage, ability damage to its physical ability scores, ability drain, energy drain, fatigue, exhaustion, or death from massive damage. It cannot be raised and resurrection only works if it is willing. It has darkvision with a range of 60 feet.

Vulnerabilities
Bone rat swarms are extremely difficult to fight with physical attacks. They have a few special vulnerabilities as follows.

A lit torch swung as an improvised weapon deals 1d3 points of fire damage per hit.

A weapon with a special ability such as flaming deals its full energy damage with each hit, even if the weapon's normal damage can't affect the swarm.

A lit lantern can be used as a thrown weapon, dealing 1d4 points of fire damage to all creatures in squares adjacent to where it breaks.

Bone Rat Swarms as Familiars
Bone rat swarms can serve as familiars for characters eligible to obtain familiars including arcane engineers, hexblades, mind weavers, shadowsworn, sorcerers, and wizards. To do so, the character must take the Improved Familiar feat, be evil in alignment, and be at least a 9^th-level caster. Powerful ratman necromancers often have bone rat swarm familiars in place of their usual undead servitor.

The bone rat swarm remains an undead creature, but gains an Intelligence score of 1. Swarm familiars do not gain the ability to speak with others of their kind, and neither the swarm nor its master gains the Skill Augmentation (Listen and Spot) feat. Instead, the following special qualities are substituted for those standard familiar abilities.

Granted Abilities: In addition to special abilities gained as a result of standard familiar progression, swarm familiars also bestow the following abilities upon their masters. The bone rat swarm's master gains immunity to her swarm's nauseating effect and its wounding attack and can occupy the same space as her familiar without suffering penalties. Swarm familiar granted abilities replace the granted abilities listed for improved familiars.

Hive Mind (Ex): Bone rat swarm familiars gain a hive mind that connects them to each other by the arcane power of their master. The master gains an empathic link and the ability to share spells with the hive mind of the swarm familiar.

Nonhealing (Ex): When a swarm takes damage, part of it is physically destroyed. Therefore, swarm familiars cannot be healed in the normal manner. Instead, new creatures much be summoned to the swam to replace those that have died. A summon swarm spell "cures" a swarm familiar by replacing dead creatures in the swarm at a rate of 1d6 hit points per caster level. The number of hit points healed in this manner cannot exceed the swarm's maximum hit point total. Only a druid or the wizard to whom the swarm familiar belongs can cure a swarm familiar in this fashion. Summon swarm spells used to heal a familiar create no other effect.

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