The Oxford Thesaurus An A-Z Dictionary of Synonyms - fateful adj.

1) significant, momentous, ominous, major, consequential, important, critical, crucial, decisive, weighty, portentous, earth-shaking, pivotal: The minister is faced with having to make a fateful decision. 2) deadly, lethal, fatal, destructive, ruinous, disastrous, catastrophic, cataclysmic: Failure to preserve the environment could have fateful consequences.