spoiler has 9 distinct synonyms spread across 2 of its 5 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.
a candidate with no chance of winning but who may draw enough votes to prevent one of the leading candidates from winning
someone who pampers or spoils by excessive indulgence
an airfoil mounted on the rear of a car to reduce lift at high speeds
a hinged airfoil on the upper surface of an aircraft wing that is raised to reduce lift and increase drag
Synonyms, antonyms and the broader and narrower relations for spoiler come from Princeton University’s WordNet. Full sources and licences.