In the Western plaguelands, the Argent Crusade is fixing everything up, we all now this. The Forsaken then decide to plague-bomb the shit outta that place. This would displease the Argent Crusade and thusly Tirion, very much. Tirion also being Alliaance helps this part of the fiction(Garrash bows to no human, remember)
Now, think as Tirion. What kind of weapon do you have at your disposal that would totaly rock the Forsakens socks off? The Ashbringer of course, but wait, what about Bolvar? Bolvars willpower is stronger than the Lich Kings, so would it be possible for him to take back control of the Valks, or maybe the new forsaken? Or if he indeed does have a lot more willpower, the original forsaken as well?