Ctesphon

Ctesphon is the King of Stygia during the reign of King Conan of Aquilonia. He held the wizard Thoth-amon in such high esteem that he exalted him above his rival magicians. For this reason, they hate and fear Thoth-amon and will seek any opportunity to destroy him.

Ctesphon casts down and elevates priests and mages at his own will in Stygia. The daughter-kingdoms of Stygia, like Kutchemes and Acheron, were priest-monarchies ruled by priest-kings. Ctesphon himself is a pureblooded white-skinned scion of the mysterious continent in the eastern ocean.

In "The Scarlet Citadel", it is revealed that Thoth's other co-conspirator, the bard Rinaldo, had dealings with Tsotha-lanthi. This one is an acolyte of the Stygian dark priest magehood and an initiate in the Stygian cult of state, the cult of Set. The Thoth-amon in "The phoenix on the Sword", the one story by Robert E. Howard where he appears, is not at all impressive, neither as a man nor as mage.