...The Daedric prince Mephala, from the Elder Scrolls. I know I am crossing lore boundaries here, but I thought it would be more appropriate to post here.

First off, the Outsider shares many characteristics with many Daedric princes. He has secretive shrines, he interferes with mortals, and is indirectly responsible for political and overall changes. But there are some things that point toward Mephala. First, Mephala's realm is one of the only Daedric realms to be obscured (undiscovered) to mortals. In the same way, the Outsider/Mephala is obscured somewhat from their own realm. This would explain why there are no beings from the Elder Scrolls universe, because nobody outisde has been there. Many things are hidden and secretive in the Isles; Magic/supernatural powers are either hidden deep in ruins or runes, or the Overseers try to cover things up. Also, Mephala is associated with elaborate plans and assassination (a common theme in the Isles). Mephala also likes to interfere with mortals, and as you know the Outsider also likes to do so.

Corvo plays the role of assassin, and Mephala had actually helped form the Morag Tong, an organization of assassins in the Elder Scrolls. The Outsider seems to know how delicate relations and situations can be. As said in the UESP, "Mephala sees the affairs of mortals as a weave; pull but one thread and the whole thing unravels." Corvo was but one thread.

I understand and respect that Dishonored and the Elder Scrolls are separate franchises, but with the mystery behind the Outsider and the thick lore of the Elder Scrolls, I saw a connection.