Originally Posted by
unholytestament
If there's lore for it. Here, let me spin something.
Following the fall of the Lich King the ghost of Sir Zeliek reappeared in his homeland (somewhere in Lordaeron?). It began appearing to certain undead warriors and priests much like how Uther's ghost appears in a few quests. He sought to help redeem some of the Forsaken and guide them back towards righteousness. A new order was formed following his example and they become a bridge between the Forsaken and the Argent Crusade.
Okay, that's not much but it could work. So long as someone comes up with a reason for why/how Sylvanas doesn't squash them before they get too powerful.