infernal has 18 distinct synonyms spread across 2 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. Its one recorded opposite is supernal.
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.
being of the underworld
“infernal regions”
an inhabitant of Hell
“his roar made the infernals quake”
2 of the 6 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for infernal.
Synonyms, antonyms and the broader and narrower relations for infernal come from Princeton University’s WordNet. Full sources and licences.