boon vs mercy

boon and mercy both are nouns.

Word	Noun	Adjective	Verb	Adverb
boon	Yes	Yes	No	No
mercy	Yes	No	No	No

As nouns, mercy is a hyponym of boon; that is, mercy is a word with a more specific, narrower meaning than boon:

boon (noun)	mercy (noun)
a desirable state	leniency and compassion shown toward offenders by a person or agency charged with administering justice
	alleviation of distress; showing great kindness toward the distressed
	a disposition to be kind and forgiving
	the feeling that motivates compassion
	something for which to be thankful

boon (adjective)	mercy (adjective)
very close and convivial