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[Warcraft] How was Illidan defeated by Arthas at Icecrown?

In Warcraft 3, there's a scene where Illidan and Arthas duel shortly before Arthas informs Illidan of the Skull of Gul'Dan. In this fight, Illidan and Arthas are perfectly matched, Illidan himself admits this. In this fight, Illidan just has the regular powers of a Demon Hunter, and Arthas is just a regular Death Knight (though the Lich King's favorite and chosen successor).

Much later, at the final battle between the Scourge and the Blood Elf and Naga army, Arthas and Illidan do battle again. At this point, however, Illidan has consumed the power contained within the Skull of Gul'Dan, and Arthas has significantly less power coming from the Lich King. But Arthas still manages to defeat Illidan. And from the cinematic, it looks like Arthas did so without much effort, with Illidan constantly on the defensive before taking a massive gash across the chest.

How was Arthas able to defeat Illidan after the balance had been tipped significantly in Illidan's favor?
gpt
Arthas isn't growing weaker during the battles leading up to Northrend, The Lich King is withdrawing his power from the Death Knight to sustain himself. In the final battle, right before Arthas and Illidan confront each other, Ner'zhul gives Arthas basically all of his power. It's basically a Hail Mary; if Arthas doesn't defeat Illidan, The Frozen throne is shattered and the Lich King dies. So the Lich King had no reason to hold back anything.