Okay, so this is the first part of three; that will cover a major part of the story.
So Anduin and Wrathion nearly make it out of Northrend, but are intercepted by Varian and Valeera, who were sent by the Lich King to recapture Anduin. A nasty fight ensues but the two are outmatched, especially Anduin who still can’t bring himself to hurt his father.
However, with Valeera’s dagger edging towards Wrathion’s heart, Anduin snaps and allows the void to fully consume his form.