Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

cataclysmic
     adj : severely destructive; "cataclysmic nuclear war"; "a
           cataclysmic earthquake" [syn: {cataclysmal}]