demoniac (adj.)

c. 1400, "possessed by a demon, insane," earlier (late 14c.) as a noun, demoniak, "one who is possessed, a lunatic," from Late Latin daemoniacus, from Greek daimoniakos "possessed by a demon," from diamon (see demon). From 1640s as "of or pertaining to demons or spirits;" by 1820 as "devilish." Related: Demoniacal; demoniacally.