The reason you won't find 'leviathine' in a dictionary is because it's not needed. Leviathan is, as I'm certain you're aware, both a noun and an adjective. The hulk was truly leviathan, for example.

Sorry to grammar-Stasi (I'm leftie, don't do Narzees) but, really, you asked.

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"Together we will build an empire of a million shining suns." -- Dave, Dictator of Utopia.