Two entities, whether they be living creatures, demons, or hostile spirits, engage in spirit combat in order to achieve some kind of dominance or influence. For a spirit this could be the habitation of a living creature which inflicts harm on the victim in the form of a disease or other malady. Demons may try to gain control of a mortal through possession.

All creatures have a hidden statistic called Power which equals its Hit Dice or Level plus the sum of its Intelligence, Wisdom, and Charisma modifiers. In each round of Spirit Combat the adversaries make Will saving throws with a DC equal to 10 + the Power of their opponent. A failed saving throw drains 1D3 Power. The first combatant to zero Power loses the contest. Power returns at a rate of one point per day but can be recovered through Restoration or Lesser Restoration spells.