WordNet records no synonym and no antonym for law of the land. 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.
a phrase used in the Magna Carta to refer to the then established law of the kingdom (as distinct from Roman or civil law); today it refers to fundamental principles of justice commensurate with due process
“the United States Constitution declares itself to be `the supreme law of the land'”
Synonyms, antonyms and the broader and narrower relations for law of the land come from Princeton University’s WordNet. Full sources and licences.