He gives the Hero instructions upon arriving outside of Summitmist Manor. In short, he says after entering, he will lock the door and no one will come in or out until all of the guests have been "taken care of."
After entering the manor, he is never seen again. It is likely that Fafnir is a member at a different Dark Brotherhood Sanctuary at some other location in Cyrodiil or abroad as he states that he and the Hero "share the same Mother."
- Fafnir is the name of the Dragon in the tale Sigurd the Dragon Slayer.