The Shattered Lands
Some beings, particularly those who are unable to cross into the mortal realm on their own, take possession of mortals to enact their will on the material plane. Using the bodies of their mortal hosts, they are able to enact their plans, coordinate followers, and gain greater power and influence.
Possession is most common among demons, who possess their hosts with sheer force of will. Devils have also been known to possess individuals, though they typically do so using promises of power as a willing host is infinitely preferable to an unwilling one. Fey rarely possess the living, but when they do so it is almost always through guile and trickery.
A creature who becomes possessed retains his or her statistics, except as specified below.
A creature subject to demonic possession must succeed on a Charisma saving throw (DC 8 + the fiend’s proficiency bonus + the entity’s Charisma modifier) or be possessed. If the possessed creature submits to the possession, his or her alignment becomes the same as the possessing entity.
If the creature resists, the possessing entity may attempt to control their host by force. This functions as the Dominate Monster spell (DC 8 + the fiend’s proficiency bonus + the entity’s Charisma modifier). If the saving throw succeeds, the creature is immune to further attempts to dominate it for 24 hours.
If the possessed creature submits to possession, or while it is dominated, it gains the following benefits:
*The creature’s type and alignment change to match the possessing entity.
*The creature gains any special senses, resistances, immunities, or vulnerabilities of the possessing entity, and can speak any languages the possessing entity can.
*The possessing entity retains any spellcasting abilities it might have.
Ending possession is difficult, and potentially dangerous to the host body, as the possessing entity can firmly attach themselves to the very soul of their host. If a possessing entity is subject to a spell such as banishment that would return it to its home plane, the possessing entity has advantage on the saving throw to resist. If the possessing entity saves against such an effect, the host takes damage equal to 1d6 x the spell level.