Open the secret passage by tapping the plate in the torture chamber. You'll enter a small room south of the chamber with a Yuan Ti priest and two Elite guards. The High Torturer will ask if you've come as student or as patient. Whatever you answer, it will lead to combat, but if you choose student and answer wisely the Torturer will not summon its guard. On death, you'll find a Ring of Pain Amplification on its body which is useless.