mortal enemy vs enemy

mortal enemy and enemy both are nouns.

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

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

Other hypernyms of mortal enemy include foe.

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