Dark Brotherhood Sanctuary (Skyrim)

In Skyrim the sanctuary is located near the road heading west from Falkreath, its entrance being guarded by a door containing a skull carving. Upon your first visit - after being sent there by Astrid - the door will ask you a question, granting entrance only when the player character replies 'Silence, my brother'. The actual sanctuary consists of a cave network with multiple rooms.

There is also a Dark Brotherhood sanctuary located in Dawnstar. It is abandoned, but you will still gain access to it in a quest. The door will ask you a question, "What is the biggest illusion?", and you will answer "Innocence, my brother".