WordNet records no synonym and no antonym for siege perilous. 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.
the legendary seat at King Arthur's Round Table reserved for the knight who would find the Holy Grail; it was fatal for anyone else to sit in it
Synonyms, antonyms and the broader and narrower relations for siege perilous come from Princeton University’s WordNet. Full sources and licences.