A lesser cranium rat pack is made up of about 75 rats, an average pack about 150, and a greater pack about 300.

CRANIUM RAT CR: 1/4 XP: 100 NE Tiny magical beast (extraplanar) Init: +2; Senses: Darkvision 60 ft., low-light vision, scent; Perception +1 __________________________________ AC: 14, touch 14, flat-footed 12 (+2 Dex, +2 size) hp: 6 (1 HD) Fort +2, Ref +4, Will +1 __________________________________ Speed: 15 ft., climb 15 ft., swim 15 ft. Melee: Bite +5 (1d3-4) Space: 2-1/2 ft.; Reach: 0 ft. __________________________________ Str 2, Dex 15, Con 13, Int 3, Wis 13, Cha 3 Base Atk: +1; CMB: +1; CMD: 9 (13 vs. trip) Feats: Weapon Finesse Skills: Climb +10, Stealth +18, Swim +10; Racial Modifiers: +4 Stealth, uses Dex to modify Climb and Swim Languages: Undercommon (can't speak) __________________________________ Environment: Any land or underground Organization: Solitary, pair, nest (3-12), or minor pack (13-74) Treasure: None

  LESSER CRANIUM RAT PACK CR: 2 XP: 600 NE Tiny magical beast (extraplanar, swarm) Init: +2; Senses: Darkvision 60 ft., low-light vision, scent; Perception +8 __________________________________ AC: 14, touch 14, flat-footed 12 (+2 Dex, +2 size) hp: 19 (3 HD) Fort +4, Ref +5, Will +4 Defensive Abilities: Swarm traits; Immune: Weapon damage Weakness: Hive mind __________________________________ Speed: 15 ft., climb 15 ft., swim 15 ft. Melee: Swarm (1d6) Space: 10 ft.; Reach: 0 ft. Special Attacks: Distraction (DC 12) Spell-Like Abilities (CL 1st, concentration +0): Once every 1d4 rounds - Mind blast (60 ft. cone, Will DC 14 negates, stunned for 1d4 rounds, level 4) __________________________________ Str 2, Dex 15, Con 13, Int 8, Wis 13, Cha 8 Base Atk: +3; CMB: --; CMD: -- Feats: Iron Will, Skill Focus (Perception) Skills: Acrobatics +6, Climb +10, Perception +8, Stealth +14; Racial Modifiers: Uses Dex to modify Climb and Swim Languages: Undercommon (can't speak) SQ: Merge swarms __________________________________ Environment: Any land or underground Organization: Solitary (see below) Treasure: None

AVERAGE CRANIUM RAT PACK CR: 5 XP: 1,600 NE Tiny magical beast (extraplanar, swarm) Init: +6; Senses: Darkvision 60 ft., low-light vision, scent; Perception +15 __________________________________ AC: 20, touch 20, flat-footed 12 (+2 Dex, +6 dodge, +2 size) hp: 52 (8 HD) Fort +7, Ref +8, Will +3 Defensive Abilities: Reflexive dodge, swarm regeneration, swarm traits; Immune: Weapon damage; Resist: Cold 10 Weakness: Hive mind __________________________________ Speed: 15 ft., climb 15 ft., swim 15 ft. Melee: Swarm (3d6) Space: 10 ft.; Reach: 0 ft. Special Attacks: Distraction (DC 15) Spell-Like Abilities (CL 4th, concentration +6): Once every 1d4 rounds - Mind blast (60 ft. cone, Will DC 16 negates, stunned for 2d4 rounds, level 4) Spells Known (CL 4th, concentration +6): 2nd (4/day) - Mirror image 1st (7/day) - Charm person (DC 13), expeditious retreat, magic missile 0 (at will) - Daze (DC 12), detect magic, flare (DC 12), mage hand, open/close, prestidigitation __________________________________ Str 2, Dex 15, Con 13, Int 14, Wis 13, Cha 14 Base Atk: +8; CMB: --; CMD: -- Feats: Combat Casting, Improved Initiative, Iron Will, Skill Focus (Perception) Skills: Acrobatics +13, Climb +10, Perception +15, Spellcraft +10, Stealth +21, Swim +10; Racial Modifiers: Uses Dex to modify Climb and Swim Languages: Undercommon (can't speak); telepathy 100 ft. SQ: Merge swarms __________________________________ Environment: Any land or underground Organization: Solitary (see below) Treasure: None

  GREATER CRANIUM RAT PACK CR: 11 XP: 12,800 NE Tiny magical beast (extraplanar, swarm) Init: +6; Senses: Darkvision 60 ft., low-light vision, scent; Perception +32 __________________________________ AC: 27, touch 20, flat-footed 19 (+7 armor, +2 Dex, +6 dodge, +2 size) hp: 143 (22 HD) Fort +14, Ref +15, Will +10 Defensive Abilities: Psychic armor, reflexive dodge, swarm regeneration, swarm traits; Immune: Cold, weapon damage; SR 22 Weakness: Hive mind __________________________________ Speed: 15 ft., climb 15 ft., swim 15 ft. Melee: Swarm (5d6) Space: 10 ft.; Reach: 0 ft. Special Attacks: Distraction (DC 22) Spell-Like Abilities (CL 10th, concentration +14): At will - Mind blast (60 ft. cone, Will DC 22 negates, stunned for 3d4 rounds, level 8) Spells Known (CL 10th, concentration +14): 5th (3/day) - Hold monster (DC 21), mind thrust V (DC 19) 4th (6/day) - Lesser globe of invulnerability, mass pain strike (DC 18), telekinesis (DC 18) 3rd (7/day) - Clairaudience/clairvoyance, gaseous form, slow (DC 17), swarm of fangs 2nd (7/day) - Blur, daze monster (DC 18), knock, mirror image, resist energy, see invisibility 1st (7/day) - Charm person (DC 17), detect thoughts (DC 15), expeditious retreat, magic missile, psychic reading 0 (at will) - Dancing lights, daze (DC 16), detect magic, flare (DC 14), ghost sound (DC 14), mage hand, open/close, prestidigitation, read magic __________________________________ Str 2, Dex 15, Con 13, Int 19, Wis 13, Cha 19 Base Atk: +22; CMB: --; CMD: -- Feats: Combat Casting, Greater Spell Focus (enchantment), Greater Spell Penetration, Improved Initiative, Iron Will, Skill Focus (Perception), Spell Focus (enchantment), Spell PenetrationB Skills: Acrobatics +27, Climb +10, Knowledge (arcana) +26, Knowledge (local) +26, Perception +32, Spellcraft +26, Stealth +31, Swim +11; Racial Modifiers: Uses Dex to modify Climb and Swim Languages: Undercommon (can't speak); telepathy 100 ft. __________________________________ Environment: Any land or underground Organization: Solitary, infestation (2-5 swarms), or plague (6-12 swarms) Treasure: None __________________________________

Hive Mind (Ex): A cranium rat swarm has a hive mind, which makes it susceptible to mind-affecting spells. For the 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. Although the packs consist of a greater number of creatures, the merged swarms pile up on top of each other and do not take up extra space. Mind Blast (Sp): A cranium rat swarm can emit a powerful blast of pure mental energy that stuns all creatures within a 60-ft. cone. Creatures that fail a Will save against the mind blast are stunned. The save DC, length of time victims are stunned, and equivalent spell level are given in each swarm’s stat block. Cranium rats are immune to mind blasts from other creatures. This is a mind-affecting enchantment (compulsion) effect. The save DC is Charisma-based. Psychic Armor (Su): An greater cranium rat pack mentally projects a protective field of force, granting it a +7 armor bonus to AC. Reflexive Dodge (Ex): The constituent creatures of an average or greater cranium rat swarm can dodge attacks, forming holes in the swarm. It gains a +6 dodge bonus to its AC. Spells: Average cranium rat packs cast psychic spells as 4th-level psychics, and greater cranium rat packs cast psychic spells as 10th-level psychics. They do not gain access to a discipline or other psychic abilities. Swarm Regeneration (Ex): When a greater cranium rat pack is reduced to 0 hit points, it does not scatter. Instead, it becomes an average pack. Likewise, 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.

  ENCOUNTERS WITH CRANIUM RATS When designing encounters with cranium rats, remember that due to their merge swarm ability, multiple cranium rat packs can come together to form a swarm of much higher CR than your players are ready for. You may choose to ignore their merge swarm ability for the sake of building a lower-CR encounter.