Definitions of fateful:

• adjective:   of ominous significance
• adjective:   having momentous consequences; of decisive importance

  Example: "That fateful meeting of the U.N. when...it declared war on North Korea- Saturday Rev"

• adjective:   controlled or decreed by fate; predetermined
• adjective:   (of events) having extremely unfortunate or dire consequences; bringing ruin

  Example: "A fateful error"