Spirit
by Simone NeriSpirits are powerful undead beings escaped from Limbo and returned to the Material Plane to harass the living. They're extremely evil and among the nastiest of undead creatures. While nearly powerless in incorporeal form, all spirits have the ability to possess and inhabit the dead bodies or body parts of other creatures (or plants, if the spirit is an odic). In incorporeal form, a spirit looks like a misty and wickedly dark shadow floating around. A body, body part or plant possessed by a spirit has a pale, decayed appearance and often emits a sickly green, blue or purple glow.
A spirit can understand all languages and usually speaks Common and another one or two additional languages. Their ability to communicate is however limited by the body they're possessing.
COMBAT
Each undead spirit is a fearsome opponent, whose tactics vary from their type and the body they're possessing. However, they share some common characteristic and abilities that make them particularly difficult to recognise and defeat.
Daylight powerlessness (Ex): A spirit is almost totally powerless while in incorporeal form. It cannot use any spell-like ability, nor its poisonous aura or its poisonous touch. If confronted in incorporeal form, it usually tries to flee and possess a body as soon as possible.
Moreover, during the daylight hours, it cannot use its ability to possess either, and become invisible (this being an extraordinary ability, it cannot be dispelled; the spirit can however be detected with the usual means). In addition, if a spirit is possessing a body, it's immediately cast out of it at sunrise, and must resume its own incorporeal form. A spirit never stays more than one night in the same place; they travel during the day, moving from place to place to find other bodies to possess. At dusk, they regain their full abilities and begin looking for a body to possess during the incoming night.
Possess (Su): A spirit full and fearsome powers manifest only when it finds a body to possess. Each type of spirit is bound to possess a particular type of body: the druj can only possess dead body parts (size going from Diminutive to Medium), the odic can only possess inanimate plants and creatures with the plant type (size going from Small to Gargantuan), while the revenant can only possess dead bodies (size from Small to Large). See individual spirit's description for more information.
A spirit can try to possess a body laying in an adjacent square as a standard action. If the target is an inanimate object (a normal plant or vegetable, a dead body or body part), it'll be automatically successful. A spirit never tries to abandon a body possessed during the same night, until forced to do so by dawn. A spirit essence's link with the possessed body is so strong during the possession time that, if the possessed body is destroyed, the spirit's essence is drawn again in Limbo and prevented from returning to the Material Plane until it manages to escape.
The possessed body or body part acquires the undead spirit template of the appropriate type (acquiring the spirit's HD, natural armour bonus in addition to their own, base attack bonus, base saving throws, special attacks and qualities, Con, Int, Wis and Cha scores, skills and feats). The spirit can use all attack modes and extraordinary abilities the possessed body had while alive (but none of the supernatural or spell-like abilities, feats, skills or spells the victim may have had). Moreover, the following effects apply:
Druj: usually the druj choses to possess a hand, a skull or an eye, although other body parts are possible. The body part maintains the Strength of the creature it belonged when alive. An eye druj applies a -6 modifier to its Strength score (up to a minimum of 1), but gains the Weapon Finesse (eye) feat. A hand druj applies instead a +2 modifier to Strength. The possessed body part loses any attack mode it had while alive and acquires the attack mode, speed rate and the Dexterity score indicated for the druj.
Odic: unique among spirits, the odic can possess the body of inanimate plants (as above) or living creatures with the plant type. If the odic tries to possess a living plant creature, the target is entitled a Will saving throw to avoid the effect; the save's DC is 10 + 1/2 Hit Dice of the spirit + the spirit's Charisma modifier. If the victim makes her save successfully, she's immune to that spirit's possession for that night. If the save fails, the victim falls under the control of that odic until the following dawn (suffering poisonous touch, poisonous aura and unholy aura effects each round until dead or freed). The odic acquires control of the victim's body (as per magic jar), but cannot use any of the victim's abilities until she's dead.
The possessed plant maintains the attack mode it had while alive (or, for inanimate plants, gains that of an animate object of the appropriate size); the possessed plant has a speed of 0 ft. (0 squares) if the odic possessed an inanimate plant, or it maintains its normal speed (if the odic possessed a plant creature).
Revenant: the possessed body gains the following modifiers to ability scores: +6 Str, +8 Dex. If the body has prehensile arms, it gains a double primary melee claw attack (whose damage depends on size; Small size = 1d6, Medium size = 2d4, Large size = 2d6)
If the body has a mouth, it gains also a secondary melee bite attack (damage = 1d3 Small, 1d4 Medium, 1d6 Large).
If the possessed body had claw or bite attack modes with better damage ratings, the latter take precedence over the revenant's. Note that the revenant can make use of all the victim's attack modes; if the body has a primary attack other than claws or manufactured weapon, the claw attacks granted by the revenant template become secondary attacks.
For example, a worg revenant would maintain its bite as primary attack and gain no claw attack. A minotaur revenant would acquire 2 claw primary attacks, a bite secondary attack (all these from revenant template) and a gore secondary attack (from minotaur's normal attack).