The theme of demonic possession occurs only occasionally in Genre Fantasy by comparison with Horror or Supernatural Fiction. The reason is perhaps the difficulty of giving a protagonist who is possessed by a Demon, and who is accordingly passive in many of his actions, any sort of that status as protagonist which is essential to emotional identification in an escapist genre. When possession occurs, it will accordingly be as a condition which the protagonist is trying to escape, as with Jason Blood in DC's The Demon comic, or as an attribute of villains, usually minor villains like the Ravers in Stephen R Donaldson's Chronicles of Thomas Covenant the Unbeliever (1977-1983).
Actual possession by evil spirits has to be distinguished from mere subjection to their will; the holders of the various Rings in J R R Tolkien's The Lord of the Rings (1954-1955) are in some cases eaten up by Sauron's influence, but not necessarily.
Possession is often indistinguishable in many of its attributes from the process of discovery that one is an Avatar of a God or goddess, a demon or some other being. Angelica, the villainess of Elizabeth Hand's Waking the Moon (1994) chooses to become an avatar of the dark side of the Goddess – she is perceived by the viewpoint character Sweeney as a woman possessed. Angelica's sacrificial castration of her lover Oliver frees him into an androgyny where he can be a vehicle for a gentler and more nurturing version of the Goddess – it is not entirely clear how much of the original Oliver survives. The protagonist of Jack Williamson's Darker Than You Think (1948) discovers that he is the dark Messiah of a primordial race of Shapeshifters; he experiences the early stages of this transformation as a somnambulism akin to possession.
Analogy with the sf tropes of symbiosis or parasitism with alien minds sometimes produces a version of demonic possession which allows the protagonist some degree of free will. His dependency on the Sword Stormbringer, which is explicitly a demon, renders Elric, in Michael Moorcock's Elric stories (1963) in the position of constantly outwitting the sword's desire to corrupt him further by drinking, and passing on to him as the energy he needs for life, the souls not only of enemies but also of friends and lovers. Dorian Hawkmoon in Moorcock's Hawkmoon series (1967-1975) is similarly half-controlled by the jewel that the evil scientists of Granbretan have implanted in his skull.
see also: Identity Exchange.