WordNet records no synonym and no antonym for armageddon. 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.
(New Testament) the scene of the final battle between the kings of the Earth at the end of the world
any catastrophically destructive battle
“they called the first World War an Armageddon”
Synonyms, antonyms and the broader and narrower relations for armageddon come from Princeton University’s WordNet. Full sources and licences.