"calamitous" in the adjective sense
1. 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"
Source: WordNet® (An amazing lexical database of English)
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