The Foregathering Spirit attacks by moving into the body of its victim. To do this, it must have a successful hit. Treat the intended victim as Armor Class 10, ignoring his/her actual armor class, but allowing for any dexterity bonuses. Once the Spirit has moved into the body, it does 2-5 HTK of dam- age per round. After six rounds inside the victim—or if the victim is reduced to 0 HTK—absorption of the victim's spirit is complete. The monster then adds another spirit to its total.
Once the creature is inside its victim's body, any attack against the Spirit must also be considered an attack against the victim. The same roll is used, and the victim has a chance to save depending on armor class. If spells are used, the victim must save vs. spells or be affected. The victim can counterattack the Spirit inside his body by rolling a natural 20 each round, which negates the Spirit's damage for that round, and causes 2 HTK of damage to the Spirit.
That sounds more like a fear aura than an unnatural aura, by the way.If any characters encounter a Foregathering Spirit, they must immediately save vs. spells or be affected by the equivalent of a Fear spell. The affected character will flee as far and fast as possible for 2-5 rounds.
Model the possession bit on malevolence?
Hmmmm. Might simplify that a bit. Is this acceptable?
Draining Malevolence (Su): Once per round, a foregathering spirit can merge its body with a creature. This ability is similar to a magic jar spell (caster level 10th or the ghost’s Hit Dice, whichever is higher), except that it does not require a receptacle. To use this ability, the foregathering spirit tries move into the target’s space; moving into the target’s space to use the malevolence ability does not provoke attacks of opportunity. The target can resist the attack with a successful Will save (DC X). A creature that successfully saves is immune to that same foregathering spirit's malevolence for 24 hours, and the spirit cannot enter the target’s space. If the save fails, the foregathering spirit vanishes into the target’s body.
If the attack succeeds, the foregathering spirit deals 1 point Con damage to the victim per minute. A successful DC X Will save negates the damage and expels the foregathering spirit. The save DC is Charisma-based.
SRD said:Feed (Su)
When a barghest slays a humanoid opponent, it can feed on the corpse, devouring both flesh and life force, as a full-round action. Feeding destroys the victim’s body and prevents any form of raising or resurrection that requires part of the corpse. There is a 50% chance that a wish, miracle, or true resurrection spell can restore a devoured victim to life. Check once for each destroyed creature. If the check fails, the creature cannot be brought back to life by mortal magic.
A barghest advances in Hit Dice by consuming corpses in this fashion. For every three suitable corpses a barghest devours, it gains 1 Hit Die, and its Strength, Constitution, and natural armor increase by +1. Its attack bonus and saves improve as normal for an outsider of its Hit Dice, and it gains skill points, feats, and ability score improvements normally.
The barghest only advances by consuming the corpses of creatures whose Hit Dice or levels are equal to or greater than its own current total. A barghest that reaches 9 Hit Dice through feeding immediately becomes a greater barghest upon completion of the act.
I like the combination with feed! But I have to say that I prefer the slower damage; gies the victim more of a chance to get rid of the spirit. But I won't object too much if you want to increase to maybe 1/round or so.