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]

WordNet (r) 3.0 (2006):

demoniacal
    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)]

Moby Thesaurus II by Grady Ward, 1.0:

101 Moby Thesaurus words for "demoniacal":
   Draconian, Mephistophelian, Tartarean, abandoned, amok, animal,
   anthropophagous, atrocious, barbaric, barbarous, beastly,
   bellowing, berserk, bestial, bloodthirsty, bloody, bloody-minded,
   brutal, brutalized, brute, brutish, cannibalistic, carried away,
   cruel, cruel-hearted, cursed, damnable, delirious, demoniac,
   demonic, demonish, demonlike, devil-like, devilish, diabolic,
   distracted, ecstatic, enraptured, execrable, fell, feral,
   ferocious, fiendish, fiendlike, fierce, frantic, frenzied,
   fulminating, furious, ghoulish, haggard, hellborn, hellish,
   hog-wild, howling, hysterical, in a transport, in hysterics,
   infernal, inhuman, inhumane, intoxicated, mad, madding, maniac,
   murderous, ogreish, orgasmic, orgiastic, possessed, rabid, raging,
   ramping, ranting, raving, ravished, roaring, running mad, ruthless,
   sadistic, sanguinary, sanguineous, satanic, savage, sharkish,
   slavering, storming, subhuman, transported, truculent, unchristian,
   uncivilized, uncontrollable, ungodly, unhuman, vicious, violent,
   wild, wild-eyed, wild-looking, wolfish