The Twilight Sepulcher is a Nordic ruin. Karliah marks its location on the Dragonborn's map.
Note: The location is first accessible during Darkness Returns.
A Nightingale Sentinel guards the entrance to the complex, as it turns out it is the ghost of Gallus Desidenius, glad to see the Skeleton Key return, but warning about the Pilgrim's Path. At the east side of the main cave the skeleton of Nystrom together with Nystrom's Journal, cryptically reveal a few things about the Pilgrim's Path that the Dragonborn must navigate.
Progress is impeded by another five aggressive Nightingale Sentinels. Note the button next to the table in the small room with the alchemy lab, opens a secret stash.
Twilight Sepulcher Inner Sanctum
Jump into a quite deep cylindrical pit to find the remains of Anders, and read his last message: it does not bode well. Luckily, with the Skeleton Key the floor simply opens up. Use the key on the Ebonmere Lock to have both Nocturnal and Karliah appear.
Note: The Inner Sanctum can then be accessed and exited via convenient teleportation wall that exits at the entrance of the first main chamber. This means the rest of the complex cannot be revisited, though.
Items of Note