Makes me think of Wormwood in Vampire the Masquerade.
He is there because the faction war plot will require him to fight for the alliance and the nelves and the only one able to defeat him was Arthas and also because probably a lot of people will want to lock him again, so no wonder why he decied to stay and also I am sure he will learn a few new tricks with the titans
Well yeah, they'd have to pull even more bullshit to let Nathanos survive against three Night Elven powerhouses. Best way out was giving Illidan the Vindicaar treatment and conveniently ignoring the fact that his actual role in keeping Sargeras imprisoned is non-existant.
No one suspects Illidan is planning this other than Mavie. I think your looking to deep into this Kape, relax a little.
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If you read the Arthas books, Illidan was owning Arthas. Pretty much just let Arthas run a bit before he went in for the finishing blow. Arthas took advantage of Illidans over-confidence and when Nerzul empowerd him/told him to strike, Arthas got in a lucky blow.
Chronicles and the artifact stories, tells both were at equal ground, I always though the novel was more Arthas self doubt than him actually losing, even lucky blows doesn't win fights.
About his plans, doubt he will ever be a villain again, probably learned that playing nice will get him more far but also he can keep his tedency or using it to get more a positive view, I bet if he came back he will raze or enslave a bunch of orcs for his new fel army and probably a lot of alliance leaders will turn a blind eye because of the current faction war and they probably will be still bitter about the whole affair.
I'm the one who's looking too deep? You're claiming that Illidan can somehow drain power from the most powerful being we currently know about. One that is watched by several titans. One the possessed Illidan's body back in Nighthold like it was nothing. Who was about to cleave Azeroth in half and was only stopped by several beings who are also stronger than Illidan. By far. In a raid where we see that draining/corrupting Titans took time and effort from Antorus elite demons.
If they ever make Sargeras' power something that can be easily stolen, that's it, they'd ruin both him and Illidan in a single swoop.
I think your just not fully informed on the lore Kape. Illidan was never possessed in the Nighthold. What we faught was some other demon (they did not state who exactly it could be) He was also dead at the time from his defeat about a decade ago in the Black Temple/just comming back to life. His spirit, was also not in the body, Kadgar was returning it to Illidans body.
Next your also changing my argument to make your claim make more sense! It's taking all of the titans power and more to hold Sargeras in place. Thats the perfect time for Illidan to strike. I know if it were a 1v1 Illidan vs Sargeras. Sargeras would not even break a sweat.