Ironically, The Lich King has a difficult time breaking Anduin.
Despite his weak frame and lack of experience, Anduin refuses to give up hope, but the Lich King won’t back down, and even resorts to bringing in Varian to help with the torture sometimes.
However in the end, it’s Anduin’s own thoughts that bring him down.The betrayal of the light hurt him just as much as his father’s, and the shadows eagerly feed on this. His confusion and hurt are slowly twisted into anger and rage, until he finally breaks, letting them consume him.
At first the Lich King believes he has finally gained control over the young prince, and he is somewhat right, but Anduin’s power stems from the Void Lords. Dark beings who are stronger than both the Old Gods and Legion combined.
Anduin can never be truly controlled, and if the Lich King slips up even a little bit, he knows Anduin will turn around and try to tear him apart.