Fafnir
From the OblivioWiki – your guide to everything Oblivion
Fafnir is your Dark Brotherhood liaison for the quest Whodunit?. He gives you your instructions upon arriving outside of Summitmist Manor. In short, he says after you go in, he locks the door and you don't come back outside until all of the guests have been "taken care of". After you enter the house, you will never see him again in the game. It is likely that Fafnir is a member at a different Dark Brotherhood Sanctuary at some other location in Cyrodiil or abroad. (He states that he and the player "share the same Mother.")
