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human
[Warcraft] What was the Lich King's endgame?

I know that Ner'Zhul betrayed the Burning Legion, it's why he was sealed inside the Frozen Throne in the first place. He then instigated the War of the Spider to secure Northrend, and finally brought ruin to all of the Eastern Kingdoms with the Plague. This was all in preparation for the Legion's Second Invasion.

It's clear, though, that Ner'Zhul desired freedom from the Legion. After all, it was Arthas who dropped the info on the Skull of Gul'Dan to Illidan, which led to the death of Tichondrius and eventually the downfall of Archimonde's Legion.

Later, Arthas dons the Crown of Domination after defeating Illidan and saving the Scourge from total destruction. This fuses Arthas and Ner'Zhul into one being, the Lich King (making the term "Lich King" more than just a title).

The Lich King then sleeps and wakes up 6 or 7 years later (assuming a one-year gap between Vanilla WoW, Burning Crusade, and WotLK) and proceeds to stir up trouble for everyone. It's obvious he wants to bring Undeath to everyone, but why? What's the point in doing that? Does he think that doing so will make Azeroth better prepared against another invasion from the Burning Legion? If that's the case, why doesn't he just bolster everyone else's forces since it's obvious that even with the Scourge on the side of the Legion during the Third War, it was still a victory for the good guys (though it was a close victory that came at a large cost).
gpt
They did not fuse. While sitting on that throne, there was a battle of wills going on between the 3 'inhabitants' of the Lich King. Arthas, Ner'zhul, and Frostmourne. Ner'zhul lost, and ended up in a purgatory of some sort.

As for the Frostmourne/Arthas combo... Frostmourne wanted one thing. To feast on souls. It didn't have any greater agenda, and convinced Arthas to serve it's ends by playing to his.

Arthas wanted to be king. He wanted a kingdom. HIS Kingdom. He wanted subjects that were loyal to him (unlike Uther and Jaina, who refused his orders and turned his backs on them) and he wanted lands free of any enemies (like the Orcs, or the Legion) forever. He wanted his perfect kingdom to stretch across Azeroth, and all living things to serve as loyal subjects of the rightful crown. And if they wouldn't, they would be made to.