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."
- "So, the last guest finally arrives. I'll tell you what I told all the others. You go in, I lock the door. You don't come out 'til it's over. Now I'll tell you what I didn't tell everyone else. We have the same Mother, you and I. And she wants you to have this. It's the key to the house.I guess someone else has already told you the other details. Kill all the guests then leave, right? Well, you'd better get in there. Time to mingle."
- "Well, what are you waiting for. There's a party, and you're the guest of honor. All I get to do is lock the damn door. You get to have all the fun."
- Fafnir is the name of the Dragon in the tale Sigurd the Dragon Slayer.