>>715101771
Even beyond the allusions to ancient history, he clearly touched upon some esoteric elements (the fall of Atlantis, a nation named Hyperborea, various pantheons). He clearly did his research and was still able to turn it into his own.
But yeah, he was the underrepresented greatest. Arguably as influential as Tolkien. My favorite is Black Colossus. It hits every mark: some supernatural horror, the machinations of a god, a huge battle sequence, and an evil wizard. Just the best.