WordNet records no synonym and no antonym for decius. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
Emperor of Rome who was proclaimed emperor against his will; his reign was notable for his severe persecution of Christians (201-251)
Synonyms, antonyms and the broader and narrower relations for decius come from Princeton University’s WordNet. Full sources and licences.