sadden has no recorded synonym. 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 gladden.
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.
make unhappy
“The news of her death saddened me”
come to feel sad
Synonyms, antonyms and the broader and narrower relations for sadden come from Princeton University’s WordNet. Full sources and licences.