downfall has 4 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
the falling to earth of any form of water (rain or snow or hail or sleet or mist)
a sudden decline in strength or number or importance
“the fall of the House of Hapsburg”
Synonyms, antonyms and the broader and narrower relations for downfall come from Princeton University’s WordNet. Full sources and licences.