This is all I know. I wonder if they would have picked up where Morgoth/Melkor, Sauron and Saruman failed.

"I think that they went as emissaries to distant regions, east and south... Missionaries to enemy occupied lands as it were. What success they had I do not know; but I fear that they failed, as Saruman did, though doubtless in different ways; and I suspect they were founders or beginners of secret cults and "magic" traditions that outlasted the fall of Sauron."

—J.R.R. Tolkien