http://elderscrolls.wikia.com/wiki/Ancient_Shrouded_Armor
Using the Ritual Stone power in Hag's End can raise the assassin corpse. Killing it allows you to obtain the Ancient Shrouded Armor set without completing (or even starting) the Dark Brotherhood questline.
There isn't anymore explained about this and I have never raised the dead, Ritual Stone or otherwise, so I have to ask: using the Ritual Stone power near the closed chamber will cause the assassin of old's corpse to resurrect outside the closed chamber?