The Collaborative International Dictionary of English v.0.48:

Cataclysmal \Cat`a*clys"mal\, Cataclysmic \Cat`a*clys"mic\, a.
   Of or pertaining to a cataclysm.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cataclysmic
    adj 1: severely destructive; "cataclysmic nuclear war"; "a
           cataclysmic earthquake" [syn: cataclysmal,
           cataclysmic]

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "cataclysmic":
   baneful, black, calamitous, cataclysmal, catastrophic, consuming,
   consumptive, convulsive, deadly, demolishing, demolitionary,
   depredatory, desolating, destroying, destructive, devastating,
   dire, disastrous, doomful, fatal, fateful, fratricidal, grievous,
   herky-jerky, insurrectionary, internecine, jerky, nihilist,
   nihilistic, orgasmic, paroxysmal, radical, ravaging, revolutional,
   revolutionary, revulsionary, revulsive, ruining, ruinous, seismic,
   self-destructive, spasmodic, spastic, subversionary, subversive,
   suicidal, sweeping, tragic, transilient, upheaving, vandalic,
   vandalish, vandalistic, wasteful, wasting, withering, wreckful