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de·mo·ni·ac(dĭ-mō′nē-ăk′) also de·mo·ni·a·cal (dē′mə-nī′ə-kəl)
1. Possessed, produced, or influenced by a demon: demoniac creatures.
2. Of, resembling, or suggestive of a devil; fiendish: demoniac energy; a demoniacal fit.
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.]