Possessed is an Acquired Template that can be applied to any humanoid creature.

Becoming a Possessed One. Becoming a Possessed One requires that the creature be the target of a ritual, and fail a will save. Or they could just fail a special will save

Hit Dice: Drop half the base creatures HD to a minimum of 2, and raise to d12s

Speed: Increase speed by +10 feet

Armor Class: The possessed one retains the armor class of the base creature.

Base Attack: The possessed one gains a slam attack appropriate to its size, as a zombie

Damage: Manufactured weapons and natural weapons deal damage normally.

Special Attacks: A possessed one retains none of the base creature's special attacks

Special Qualities:

Saves: As the base creature and the possessing entity. See abilities.

Abilities: A possessed one has the INT, WIS, and CHA of the possessing entity. Other than that, it gains +4 STR, +2 Dex

Feats: A possessed one loses the feats of the base creature, and gains Spider Climb and Toughness.

Environment: Any. Possessed Ones may not voluntarily enter sanctified ground, or anywhere guarded by Protection from Evil, Consecrate, or Hallow.

Organization: Solitary. However multiple spirits can inhabit the same body.

CR: CR of the spirit + CR of the possessed creature. Multiple spirits do not stack--rather one spirit at a time is in control of the body.

Treasure: As the base creature

Alignment: Chaotic Evil. If the possession was not voluntary on the part of the possessed creature, it does not suffer from alignment-related effects such as the loss of spells + class abilities if the spirit leaves.

Level Adjustment: Varies

Exorcism: Need to think up rules for this