fa·tal adj
1: causing failure of a legal claim or cause of action
a fatal defect in the proceedings — W. R. LaFave and J. H. Israel
2: making something (as a contract) invalid or unenforceable
there is a fatal indefiniteness with the result that the agreement is void — J. D. Calamari and J. M. Perillo
fa·tal·ly adv
Merriam-Webster’s Dictionary of Law. Merriam-Webster. 1996.