FATAL: "Bringing death; Having momentous consequences; of decisive importance"

-Controlled or decreed by fate; predetermined
-(of events) having extremely unfortunate or dire consequences; bringing ruin

"the fatal day of the election finally arrived"

"such doctrines, if true, would be absolutely fatal to my theory"; "it is fatal to enter any war without the will to win it"

"a fatal series of events"

synonyms: disastrous, calamitous

clipped from www.scdf.gov.sg