When I was doing Liu Fei’s questline, I realized I knew the name “Mezzamalech” from somewhere else and it turned out I was right.
Liu Fei’s demon might be the same person as Zon Mezzamalech from Clark Ashton Smith’s “Ubbo-Sathla” short story. He was a powerful Hyperborean (Hyperborean Age is to Thurian Age, what Thurian Age is to our modern times) sorcerer, who possessed a mystical crystal, that allowed him to view past and future and collect knowledge.
Apparently he went so far back in time, that he managed to scry upon Ubbo-Sathla, the prime source of all living things on Earth. The entity was guarding a set of tablets containing the secrets of an extinct godlike race. He tried to read them and, according to Book of Eibon, disappeared into the parts unknown.
Judging from the questline details, he managed to get his knowledge all right. Judging from the upgrade to the status of a demon.
Apparently Ubbo-Sathla exists in Conan Exiles verse, since in the volcano we can find and fight a mini boss named Emmissary of Abhoth, Abhoth being Ubbo-Sathla’s progeny.