Thothmekri is a ancient High Priest of Set in "The Hour of the Dragon". Thothmekri lived three thousand years ago, making him a contemporary of the sorcerer Xaltotun. He was an adept of the Black Ring.
After he was being resurrected in the Temple of Set by his followers with the Heart of Ahriman, he showed the handsome Cimmerian warrior-king Conan the exit out of the ancient tombs of Luxor, and then he returned. Whether he laid back down in eternal slumber just like the beautiful Stygian vampire princess Akivasha did before him, or he wanders those tombs as one of the living undead corpses guarding her private royal tomb still remains unanswered until right now.
Not much is known about his bitter rival named Thothmekri, other than he was an adept of the Black Ring, and since he was chosen to be resurrected by Thutothmes, then perhaps it was assumed that he was incredibly powerful. Thutothmes presumed that Thothmekri had the knowledge of the true workings of the Heart of Ahriman.
This one is doubtful, for none like them in Xaltotun's time really knew the powers of the Heart, but the people must assume that Thutothmes had done his homework.
"He towered beside the altar, dusky, naked, with a face like a carved image. Conan realized that the conjurement had not been completed - life had not been fully restored to the corpse. 'Who are you?' demanded the Cimmerian. The answer came in a toneless monotone, like the dripping of water from stalactites in subterranean caverns. 'I was Thothmekri; I am dead.'" -- Robert E. Howard, "The Hour of the Dragon"