"Many have sought to harness the corruptive magic of the Void. Most who tried have fallen into madness. Determined to use this power for the good of Azeroth, Alleria Windrunner is the first mortal to succeed at defying the shadow's whispers. Coming to the aid of a group of her kin who nearly gave in to the darkness, Alleria has vowed to train these Void Elves to control the shadows within them and pledge their newfound powers to the Alliance."
We know The blood elves are going to lose Quel'thalas and the Eastern kingdoms will belong to the Alliance. So from the description it seems that Quel'thalas will come under void corruption, Blood Elves will be forced to abandon it and Alleria will come to the rescue of her kin who were left behind. Does this sound likely? Something big is about to happen.