Source : Webster's Revised Unabridged Dictionary (1913)

Demoniac \De*mo"ni*ac\, n.
   1. A human being possessed by a demon or evil spirit; one
      whose faculties are directly controlled by a demon.

            The demoniac in the gospel was sometimes cast into
            the fire.                             --Bates.

   2. (Eccl. Hist.) One of a sect of Anabaptists who maintain
      that the demons or devils will finally be saved.

Demoniac \De*mo"ni*ac\, Demoniacal \Dem`o*ni"a*cal\ (?; 277), a.
   [L. daemoniacus, fr. daemon; cf. F. d['e]moniaque. See
   {Demon}.]
   1. Pertaining to, or characteristic of, a demon or evil
      spirit; devilish; as, a demoniac being; demoniacal
      practices.

            Sarcastic, demoniacal laughter.       --Thackeray.

   2. Influenced or produced by a demon or evil spirit; as,
      demoniac or demoniacal power. ``Demoniac frenzy.''
      --Milton.

Source : WordNet®

demoniac
     adj 1: in a murderous frenzy as if possessed by a demon; "the
            soldier was completely amuck"; "berserk with grief";
            "a berserk worker smashing windows" [syn: {amuck}, {amok},
             {berserk}, {demoniacal}, {possessed(p)}]
     2: resembling or suggesting possession by a demon; "demoniac
        energy"; "a demoniacal fit" [syn: {demoniacal}]

demoniac
     n : someone who acts as if possessed by a demon