system
Given the following post, respond as if you're commenting on Reddit:
human
[Warcraft] So where exactly did the Lich King go wrong?

I'm posing this question in regards to the events surrounding the Wrath of the Lich King expansion. He had enormous resources, ample time to plan, and was himself incredibly powerful. What did he not account for that saw his conquests thwarted? What could he have done differently to succeed?
gpt
He let the wrong enemies get too close.  It was part of his plan to turn his citadel into a trial by fire; those heroes who survived to the end would be converted into servants of his will.  And this plan to turn his enemy's strength into his own might have worked.  However, thanks to the Light, the paladin Tirion Fordring was able to free himself from the Lich King's grasp at the last moment and use the souls trapped in the Lich King's sword against him.  Certain victory for the Lich King was turned into a sudden defeat owing to Fordring's presence.

So, arguably, the Lich King's mistakes were (1) letting Fordring onto the Frozen Throne and (2) letting him live.  If he had avoided either one of those mistakes, his plan could have gone off without a hitch.  Instead, though, the Lich King was very intent on making Fording live long enough to witness and suffer through his victory.  In that sense it was sort of a typical villain mistake.