A Sanctuary Guardian is a ghostly apparition of a deceased assassin of the Dark Brotherhood, enthralled to protect a specific place. This will most often be a Sanctuary of the Brotherhood, particularly one that has been closed or abandoned. These ghostly assassins only recognize individuals who possess some sort of password to gain their trust and will attack all others, including family members. It is not known if this is because these ghosts are no longer truly sentient or because they are only in fact summoned shades.
The abandoned Sanctuary in the north of Skyrim was known to certain members of the Brotherhood to contain a contingent of such protectors.