severe has 13 distinct synonyms spread across 5 of its 6 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.
unsparing and uncompromising in discipline or judgment
“a parent severe to the pitch of hostility”
causing fear or anxiety by threatening great harm
“a dangerous operation”
1 of the 6 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for severe.
Synonyms, antonyms and the broader and narrower relations for severe come from Princeton University’s WordNet. Full sources and licences.