The other day I was flipping through some anthologies of horror stories and I came upon Sir Arthur Conan Doyle's The Ring of Thoth. This story is generally considered the primary influence for the 1932 horror film, The Mummy, but as I gave it a re-read I began to wonder if it might also have been influential in Robert E. Howard's creation of the Stygian sorcerer Thoth-Amon who appears in the first Conan story The Phoenix on the Sword.
The name Thoth is the main tip off of course, but there's another possible connection. When we first encounter Thoth-Amon, the once mighty sorcerer has lost a ring which held much of his sorcerous power and is now a slave, though actively searching for his ring. In Doyle's story, the 4000 year old Egyptian Sosra is working as a lowly attendant in the Louvre while he seeks a lost ring which contains the power to end his artificially extended life. It's not a close parallel, but it is suggestive.
So once again I went back to the inestimable REH Bookshelf at Rehupa, and checked the listing for Sir Arthur Conan Doyle. REH owned three volumes of the Conan Doyle's Best Books series including one which contained The Ring of Thoth. So it's likely that he read the story. (Howard's library contained a lot of Conan Doyle's work, which as a Sherlockian, always makes me happy.)
I also wonder if The Ring of Thoth might have been an influence on the titular character in Skull Face, who while based primarily on Fu Manchu, is an ancient and mummy-like being. Sosra has a face like parchment and strange eyes in deep sockets. Someone points out that Sosra's features aren't like those of modern men, a trait that Kathulos shares. There are a couple of other sorcerers in the works of REH who have mummy-like characteristics as well.
Anyway, it's always fun to speculate on the antecedents of an author's work. No one works in a vacuum and all writers are influenced by what they read. Of course, as others have pointed out, no matter what Howard borrowed, he always made it uniquely his own by the sheer force of his writing.