The Collaborative International Dictionary of English v.0.48:

Havoc \Hav"oc\ (h[a^]v"[o^]k), n. [W. hafog devastation, havoc;
   or, if this be itself fr. E. havoc, cf. OE. havot, or AS.
   hafoc hawk, which is a cruel or rapacious bird, or F. hai,
   voux! a cry to hounds.]
   Wide and general destruction; devastation; waste.
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         As for Saul, he made havoc of the church. --Acts viii.
                                                  3.
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         Ye gods, what havoc does ambition make
         Among your works!                        --Addison.
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The Collaborative International Dictionary of English v.0.48:

Havoc \Hav"oc\, v. t.
   To devastate; to destroy; to lay waste.
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         To waste and havoc yonder world.         --Milton.
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The Collaborative International Dictionary of English v.0.48:

Havoc \Hav"oc\, interj. [See Havoc, n.]
   A cry in war as the signal for indiscriminate slaughter.
   --Toone.
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         Do not cry havoc, where you should but hunt
         With modest warrant.                     --Shak.
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         Cry 'havoc,' and let slip the dogs of war! --Shak.
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WordNet (r) 3.0 (2006):

havoc
    n 1: violent and needless disturbance [syn: havoc, mayhem]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "havoc":
   abomination, atrocity, bad, bane, befoulment, blight, bloodbath,
   blue ruin, breakup, calamity, carnage, cataclysm, catastrophe,
   chaos, confusion, consumption, corruption, crying evil, damage,
   damnation, decimation, defilement, depredate, depredation,
   desecrate, desolate, desolation, despoil, despoiling, despoilment,
   despoliation, destruction, detriment, devastate, devastation,
   disintegration, disorder, disorganization, disruption, dissolution,
   evil, grievance, harm, harry, hecatomb, holocaust, hurt, ill,
   infection, injury, loss, mayhem, mischief, outrage, perdition,
   pillage, pillaging, poison, pollution, ravage, ravaging, ruin,
   ruination, sack, shambles, slaughter, spoliate, spoliation,
   the worst, toxin, undoing, upset, vandalism, venom, vexation,
   waste, woe, wrack, wrack and ruin, wreck, wrong