mercy vs mildness

mercy and mildness both are nouns.

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

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

Other hyponyms of mercy include compassion, pity, forgivingness, kindness, lenience, leniency, lenity.

mercy (noun)	mildness (noun)
leniency and compassion shown toward offenders by a person or agency charged with administering justice	mercifulness as a consequence of being lenient or tolerant
alleviation of distress; showing great kindness toward the distressed	acting in a manner that is gentle and mild and even-tempered
a disposition to be kind and forgiving	good weather with comfortable temperatures
the feeling that motivates compassion
something for which to be thankful