Ineffable Horror (CR 5)

Large Aberration
Alignment: Always chaotic evil
Initiative: +1 (Dex); Senses: darkvision 60 ft., Listen +4, and Spot +4
Languages: Undercommon

AC: 18 (-1 size, +1 Dex, +8 natural), touch 10, flat-footed 17
Hit Dice: 5d8+15 (37 hp)
Fort +4, Ref +2, Will +4
Speed: 30 ft., fly 60 ft. (good)
Space: 10 ft./10 ft.
Base Attack +3; Grapple +11
Attack: Slam +7 melee (1d4+4)
Full Attack: 2 slams +7 melee (1d4+4)
Damage: -
Special Attacks/Actions: Attaching intestines, blood drain, improved grab
Abilities: Str 19, Dex 13, Con 16, Int 8, Wis 10, Cha 4
Special Qualities: SR 16
Feats: Blind-fight; Weapon Focus (slam)
Skills: Listen +4 and Spot +4
Advancement: 6-10 HD (Large); 11-15 HD (Huge)
Climate/Terrain: Underground (Lower and Middle Underdark
Organization: Solitary, pair, or murder (3-12)
Treasure/Possessions: Standard

Source: Underdark

Attaching Intestines (Ex): Upon a successful grapple check, an ineffable horror's writhing intestines latch onto the grappled opponent's body. Attached intestines can be removed only by escaping the grapple.

Blood Drain (Ex): An ineffable horror's attached intestines drain blood, dealing 1d4+1 points of Constitution damage each round that the monster remains attached to its prey. Creatures immune to critical hits are immune to the monster's blood drain ability. The ineffable horror usually does not cease its blood drain until its victim dies.

Improved Grab (Ex): If an ineffable horror hits an opponent its own size or smaller with a slam attack, it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity (grapple bonus +11). If it gets a hold, its, intestines latch onto its foe (see Attaching Intestines above).

An ineffable horror usually relies upon its physical prowess to grab a foe and drain it of blood. It is a tenacious combatant that prefers to attack the same target until either it or its victim is dead. Ineffable horrors frequently travel in large flocks, called murders, and use group tactics (particularly flanking) to their advantage.