living dead vs decedent

living dead and decedent both are nouns.

Word	Noun	Adjective	Verb	Adverb
living dead	Yes	No	No	No
decedent	Yes	No	No	No

As nouns, decedent is a hypernym of living dead; that is, decedent is a word with a broader meaning than living dead:

living dead: a dead body that has been brought back to life by a supernatural force
decedent: someone who is no longer alive

Other hypernyms of living dead include dead person, dead soul, deceased, deceased person, departed.

living dead (noun)	decedent (noun)
a dead body that has been brought back to life by a supernatural force	someone who is no longer alive