mercifulness has 2 distinct synonyms spread across 3 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is mercilessness.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a disposition to be kind and forgiving
“in those days a wife had to depend on the mercifulness of her husband”
Synonyms, antonyms and the broader and narrower relations for mercifulness come from Princeton University’s WordNet. Full sources and licences.