Possessed, produced, or influenced by a demon: demoniac creatures.
Of, resembling, or suggestive of a devil; fiendish: demoniac energy; a demoniacal fit.
n. One who is or seems to be possessed by a demon.
[Middle English demoniak, from Late Latin daemoniacus, from Greek *daimoniakos, from daimonios, of a spirit, from daimōn, divine power; see demon.] de'mo·ni'a·cal·ly adv.