I'd guess it'd be a lesser Daedra. They'd be the most likely in the Deadlands to be punished, and since they're immortal, it'd be teaching them a lesson that they will be in a position to learn.
Given that Mehrunes Dagon has shown that he can and will grant eternal life to mortals
Spoiler Mankar Camoran's Paradise
, it's not unthinkable that they started out as human beings, although I prefer to believe they might have been Mythic Dawn followers rather than adventurers, as there are simply too many to be caused by hapless adventurers so soon after the Siege of Kvatch.