demoniac

adj

1. Possessed or controlled by a demon.
2. Of or pertaining to demons; demonic.

   “How dark may be the hiding of God's face, Or what demoniac forms may seize the helm Of reason...”

   “Animal fury and orgiastic licence here whipped themselves to demoniac heights by howls and squawking ecstasies that tore and reverberated through those nighted woods like pestilential tempests from the gulfs of hell.”

   “There was movement everywhere, screaming, demoniac activity; the old man was coming across the tumbling logs.”

noun

1. Someone who is possessed by a demon.

   “The exorcism was dropped from the second Edwardian Prayer Book, because of its implication that unbaptised infants were demoniacs […].”