I don't think that necessitates how they work him into the story, though. I think there are plenty of ways of making an interesting new villain without actually tying him in to preexisting plots at all—he could simply appear, he could be something made from a loose thread (i.e. the idea of a fifth Old God eaten by the others has some background), he could be made in front of us from a character as we see them become a villain (like Garrosh was, who I honestly think is one of the best villains).
Besides, I suppose we do need more villains to be made, since there are so many who have already been around.
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That's because there was a mystery around them being absent before. People seem excited about Turalyon. You are the only person I have ever seen excited about the Void Elves. Plenty of people play them, but I've rarely seen someone with your unhealthy fixation on this plot.
And just say "Void Elves". You are not an Elf in World of Warcraft. You are a human being on Earth. Write "Void Elf".