Reason Stealer (CR 5)

Medium Ooze
Alignment: Always neutral
Initiative: -5 (Dex); Senses: blindsense 60 ft.
Languages: No language

AC: 5 (-5 Dex), touch 5, flat-footed 10
Hit Dice: 5d10+15 (42 hp)
Fort +4, Ref -4, Will -4
Speed: 10 ft., climb 10 ft.
Space: 5 ft./5 ft.
Base Attack +3; Grapple -2
Attack: Slam +5 melee, or weapon +5 melee and slam +0 melee
Full Attack: Slam +5 melee, or weapon +5 melee and slam +0 melee
Damage: Slam 1d6+3 (primary), by weapon +3, slam 1d6+1 (secondary)
Special Attacks/Actions: Devour mind, improved grab
Abilities: Str 14, Dex 1, Con 16, Int -, Wis 1, Cha 1
Special Qualities:
Feats: -
Skills: -
Advancement: 6-10 HD (Medium-size); 11-15 HD (Large)
Climate/Terrain: Any underground
Organization: Solitary
Treasure/Possessions: None

Source: Monster Manual II

Devour Mind (Su): A reason stealer transfers an opponent's Intelligence score to itself upon dealing it a killing blow with a slam attack. This process also heals the monster of 5 points of damage per point of Intelligence gained. Upon devouring a mind, the reason stealer has access to any of the dead opponent's other ability scores, skill ranks, base attack bonus, and base save bonuses, and it can use any of these characteristics that are higher than its own. It also gains the use of the opponent's feats and any prepared arcane spells. The reason stealer understands that it must remove spell components from a victim's body to cast spells. All these acquired features last for 24 hours. If a reason stealer devours the mind of a victim while it still has the abilities of a previous one, it uses the higher of the two (the new victim's or its current value) for ability scores, base attack bonus, base save bonuses, and skill ranks. It also gains any feats and spells possessed by the new victim that it did not already have.

Improved Grab (Ex): If a reason stealer hits an opponent that is 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 +9, including a +4 racial bonus on grapple attacks from the sticky substance that covers its pseudopod). If it gets a hold, it has the option to conduct the grapple normally, or simply use its pseudopod to hold the opponent (-20 penalty on grapple check, but the reason stealer is not considered grappled). In either case, each successful grapple check it makes during successive rounds automatically deals slam damage.

Blindsight (Ex): A reason stealer is blind, but its entire body is a primitive sensory organ that can ascertain prey by scent and vibration. This ability enables it to discern objects and creatures within 60 feet. A reason stealer usually does not need to make Spot or Listen checks to notice creatures within range of its blindsight.

Grotesque Form (Su): A reason stealer that has devoured the mind of a creature with an Intelligence score of 1 or higher can shape its body into a bizarre reflection of that creature. If the reason stealer has abilities gained from more than one creature, it assumes a form similar to the last one it killed. A reason stealer with a humanoid form can wield weapons, using the fighting skills of any victims it devoured. It always retains one pseudopod with which to attack. A reason stealer can maintain a grotesque form as long as it has an Intelligence score.

A reason stealer lashes out at prey with a sticky pseudopod. Upon delivering the death blow to a foe with its pseudopod, it steals the opponent's intellect. Once a reason stealer gains a modicum of sentience, it becomes a far deadlier opponent because it can use all the skills, feats, and spells it has absorbed from its prey. Sentient reason stealers ceaselessly search for new prey because they are desperate to avoid returning to their previous mindless state.