fatal has 4 distinct synonyms spread across 3 of its 4 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is nonfatal.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
bringing death
having momentous consequences; of decisive importance
“that fateful meeting of the U.N. when...it declared war on North Korea”
(of events) having extremely unfortunate or dire consequences; bringing ruin
“the stock market crashed on Black Friday”
Synonyms, antonyms and the broader and narrower relations for fatal come from Princeton University’s WordNet. Full sources and licences.