WordNet sense 1 ((of events) having extremely unfortunate or dire consequences; bringing ruin):

black, calamitous, disastrous, fateful

WordNet sense 2 (causing or ending in or approaching death):

terminal

WordNet sense 3 (of an instrument of certain death):

deadly

WordNet sense 4 (causing or capable of causing death):

mortal

WordNet sense 5 (determining or having the power to determine an outcome):

decisive

WordNet sense 6 (incapable of being avoided or prevented):

inevitable