Meaning : (of events) having extremely unfortunate or dire consequences. Bringing ruin.
Example :
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.
Synonyms : black, calamitous, fatal, fateful