mercy vs succour

mercy and succour both are nouns.

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

As nouns, succour is a hypernym of mercy; that is, succour is a word with a broader meaning than mercy:

mercy: alleviation of distress; showing great kindness toward the distressed
succour: assistance in time of difficulty

Other hypernyms of mercy include ministration, relief, succor.

mercy (noun)	succour (noun)
leniency and compassion shown toward offenders by a person or agency charged with administering justice	assistance in time of difficulty
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

mercy (verb)	succour (verb)
	help in a difficult situation