[Rune Son of Bjarne]

Is it Sauron then?

It would sertainly explain "die by my own hands" as he forged the one ring and when it is destroyed he dies. . . maybe a strech, but I think it works.

He has been around for a long while
He has supported Morgoth, but not from the begining.

The treachery could also be when the Numenorians attack aman. . .and Sauron did his part there as werll. . .that would add perfectly up when they come to undo treachery.