[octothorp]

Unlikely but thematically rich interpretation for the Dark Wizard: Rhun is near where the elves first were put on middle earth at Cuiviénen, and there was a company of Maiar who were to guard them. Eventually the elves would split with most travelling West. Of those that stayed behind, this is where Morgoth originally captured the elves that he twisted into Orcs. That includes Adar. So suppose the Dark Wizard is a Maiar that stayed in those lands after most of the elves departed, and was unable to protect those that remained from Morgoth. He stayed through the first age, stayed when Cuiviénen was destroyed during the changing of the world at the beginning of the second age. Maybe when men start arriving in those lands, he originally sees himself as a protector of them, but then that changes into a desire to rule over them, for their protection. The Valar become aware that there's a forgotten Maiar in Rhun, and send Gandalf... a Marlow to this other Maiar's Kurtz.

Highly unlikely, but I'd love that direction so much more than the dark wizard being one of the other five istari from the third age.