WordNet records no synonym and no antonym for reign of terror. It is the only member of each of its synsets, 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.
any period of brutal suppression thought to resemble the Reign of Terror in France
the historic period (1793-94) during the French Revolution when thousands were executed
“the Reign of the Bourbons ended and the Reign of Terror began”
Synonyms, antonyms and the broader and narrower relations for reign of terror come from Princeton University’s WordNet. Full sources and licences.