de·mo·ni·ac 
(dĭ-mōnē-ăk′) also de·mo·ni·a·cal (dē′mə-nīə-kəl)
(dĭ-mōnē-ăk′) also de·mo·ni·a·cal (dē′mə-nīə-kəl)adj.
1. Possessed, produced, or influenced by a demon: demoniac creatures.
2. 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·nia·cal·ly adv.