Well.... It was that or have alot of dead horde leaders xD
And she was not the supreme leader, Vol'jin was. She could do it because she was the only one to have moving troops left in the form of the lesser val'kyrs.
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I really hope, that Blizz next expansion will put focus on the Horde and Alliance again. Not as in direct conflict, but as moving and acting chess pieces in a cold war, that sometimes becomes a bit hot here and there.
May the lore be great and the stories interesting. A game without a story, is a game without a soul. Value the lore and it will reward you with fun!
Don't let yourself be satisfied with what you expect and what you seem as obvious. Ask for something good, surprising and better. Your own standards ends up being other peoples standard.
Warchief Sylvanas right? i think you misspelled War.
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Not sure if Grom will show up anytime soon, would seem a bit far-fetched. I also feel like blizz wants to leave the that in the back burner. They left it open ended and i think we are beyond the point to justify an intervention from the Alternate Universe.
As for Sylvanas, why would she be part of a wrath 2 expac? Arthas is dead.