I am going to be very vague here but Tirion Fordring is just one of the people The Lich King is considering for the fourth horseman. There’s a reason I say this is the “Second Worst Idea Tev Has Ever Heard.”
Actually I’m not sure if what happened wasn’t his plan the entire time, come to think of it. He split the Ashen Verdict and now one of the people who had questioned him has bee raised by him and might be under his thumb again?