decedent vs living dead

decedent and living dead both are nouns.

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

As nouns, living dead is a hyponym of decedent; that is, living dead is a word with a more specific, narrower meaning than decedent:

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

Other hyponyms of decedent include infernal, zombi, zombie.

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