Main entry: black, calamitous, fatal, fateful, disastrous
Definition: (of events) having extremely unfortunate or dire consequences; bringing ruin
Usage: 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