mortal enemy vs foe

mortal enemy and foe both are nouns.

Word	Noun	Adjective	Verb	Adverb
mortal enemy	Yes	No	No	No
foe	Yes	No	No	No

As nouns, foe is a hypernym of mortal enemy; that is, foe is a word with a broader meaning than mortal enemy:

Other hypernyms of mortal enemy include enemy.

mortal enemy (noun)	foe (noun)
an enemy who wants to kill you	a personal enemy
	an armed adversary (especially a member of an opposing military force)