WordNet: black, calamitous, disastrous, fatal, fateful

(of events) having extremely unfortunate or dire consequences; bringing ruin; "the stock market crashed on Black Friday"; "a calamitous defeat"; "the battle was a disastrous end to a disastrous campaign"; "such doctrines, if true, would be absolutely fatal to my theory"- Charles Darwin"; "it is fatal to enter any war without the will to win it"- Douglas MacArthur"; "a fateful error"