fatal

"that fateful meeting of the U.N. when...it declared war on North Korea"; "the fatal day of the election finally arrived";

"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"; "it is fatal to enter any war without the will to win it"; "a fateful error";

"a fatal series of events";