"Legion of the Dead"
The Devourer of Souls has captured a child that it senses holds great mystic power that he intends to steal. The child bears a cat, and the Devourer cannot divine the secret between them, so he takes them to the Land of the Dead and consults with the spirit of Thulsa Doom. Doom relates a tale from the time of King Kull, about Doom's attempts at Kull's life and his cat's paw, a boy named Sedrick who betrayed him and was turned by Doom into a cat, the cat that the child holds. Now that the Devourer knows, he intends to steal the child's power to crush Conan, who is on his way with a band of followers that includes Red Sonja to stop the Devourer. Strange alliances are made, and Thulsa Doom fights beside Conan and Red Sonja.
Story continues in Conan the Barbarian (1970) #200.