deathly vs mortal

deathly is not a noun while mortal is a noun.

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

As adjectives, deathly and mortal are synonyms defined as:

Other synonyms of deathly include deadly.

deathly (noun)	mortal (noun)
	a human being; person, singular, assertive existential pronoun; pronoun, person, singular; quantifier: assertive existential

deathly (adjective)	mortal (adjective)
having the physical appearance of death	subject to death
causing or capable of causing death	unrelenting and deadly
	involving loss of divine grace or spiritual death
	causing or capable of causing death

deathly (adverb)	mortal (adverb)
to a degree resembling death
to an extreme degree