Unfortunately, the path is being monitored by the Deathguards, or mourners, so you'll first need to disguise yourself with a set of mourner gear. With the camouflage equipped or in your inventory, speak to Arianwyn again to teleport to the Corridor of Light. Head west across the corridor and enter the door at the end to emerge in a storeroom. Continue past another door into the next room, where the Deathguard captain will demand to know what you were doing in the storeroom - if you answer any questions incorrectly, Arianwyn will teleport you both back to the Corridor of Light.