The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            Sarcastic, demoniacal laughter.       --Thackeray.
      [1913 Webster]

   2. Influenced or produced by a demon or evil spirit; as,
      demoniac or demoniacal power. "Demoniac frenzy." --Milton.
      [1913 Webster]

   3. resembling or suggesting possession by a demon; as,
      demoniac energy.
      [WordNet 1.5]

   4. in a murderous frenzy as if possessed by a demon.

   Syn: amuck, amok, berserk, demoniac, possessed(predicate).
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            The demoniac in the gospel was sometimes cast into
            the fire.                             --Bates.
      [1913 Webster]

   2. (Eccl. Hist.) One of a sect of Anabaptists who maintain
      that the demons or devils will finally be saved.
      [1913 Webster]

WordNet (r) 3.0 (2006):

demoniac
    adj 1: frenzied as if possessed by a demon; "the soldier was
           completely amuck"; "berserk with grief"; "a berserk
           worker smashing windows" [syn: amuck, amok,
           berserk, demoniac, demoniacal, possessed(p)]
    n 1: someone who acts as if possessed by a demon