memorial vs empty tomb

memorial and empty tomb both are nouns.

Word	Noun	Adjective	Verb	Adverb
memorial	Yes	No	No	No
empty tomb	Yes	No	No	No

As nouns, empty tomb is a hyponym of memorial; that is, empty tomb is a word with a more specific, narrower meaning than memorial:

memorial: a structure erected to commemorate persons or events
empty tomb: a monument built to honor people whose remains are interred elsewhere or whose remains cannot be recovered

memorial (noun)	empty tomb (noun)
a structure erected to commemorate persons or events	a monument built to honor people whose remains are interred elsewhere or whose remains cannot be recovered
a recognition of meritorious service
a written statement of facts submitted in conjunction with a petition to an authority