CONAN has lost his lady friend in a dungeon below an ancient city...

CONAN has lost his lady friend, Natala, in a dungeon below an ancient city. Thalis, who is competing with Natala for CONAN's attention, overpowers her and gets a bit lesbo-sadomasochistic on her arse!

As in a nightmare Natala felt her tunic being stripped from her, and the next instant Thalis had jerked up her wrists and bound them barely touching the floor. Twisting her head, Natala saw Thalis unhook a jewel-handed whip from where it hung on the wall, near the ring. The lashes consisted of seven round silk cords, harder yet more pliant than leather thongs.
With a hiss of vindictive gratification, Thalis  drew back her arm, and Natala shrieked as the cords curled across her loins. The tortured girl writhed, and tore agonizedly at the thongs which imprisoned her wrists.
...She had never guessed the punishing power of hard-woven silk cords. Their caress was more exquisitely painful than any birch twig or leather thongs.

From, "The Slithering Shadow"
Page 151-2 "The Complete Chronicles of CONAN"

It's quite interesting that although there have been many instances of cruelty by men upon women being mentioned, the only graphic description I have found so far decribes a woman taking pleasure in cruelty to another woman.