Dark Brotherhood Sanctuary is a location added by Wyrmstooth.

At the eastern side of the island, there is an abandoned Dark Brotherhood Sanctuary. Its door, unlike the vanilla doors, does not request a secret password. The Dragonborn will only find one dead body and several chests with some not so useful loot. Near the entrance, there is a small room with a Draugr. In the first level of the sanctuary, a note will warn about more Draugr downstairs.

It seems that the sanctuary was abandoned, but the brotherhood had left a single guard: the dead assassin.

After defeating the draugr there is a treasure room with a safe, an empty display case, a lockbox, and other various loot. Also there is manure (ingredient) located either in the loot room or alchemy room. Strangely torture tools (unique item) can be found in the initiate area.