I'm an currently in Snow Veil Sanctum and I'm having trouble finishing the Speaking with Silence quest. During this quest I'm supposed to lead Mercer Frey into Snow Veil Sanctum to kill Karliah, a former Thieves Guild initiate. I've already made my way to the end of the ruins where I battle a multitude of Draugs and learn the 'weapon, disarm' word. Unfortunately, I still can't get to Karliah because she's behind a door that requires a claw to open. I've killed everything up to this point and nothing has dropped a claw. Where am I supposed to get this thing? Is it in the ruins somewhere?