From Longman Dictionary of Contemporary Englishmortal enemy/foemortal enemy/foeENEMYan enemy that you hate very much and will always hate He glared at Claudia as if she were his mortal enemy. → mortal
Examples from the Corpus
mortal enemy/foe• Well, it might be a big one with a misleading number-plate which simply doesn't stop that gets your mortal enemy.• It was also the mortal enemy of horses and would rip them apart in seconds with its mighty talons.• I remembered doing this sort of stuff in sixth grade with my mortal enemy, Tommy Jancko. 