Didn't those events take place before his exile? When the Alliance and Horde were still officially at war?

I admit that my memory of the events isn't the best, but if I recall correctly, he was exiled at a later date, upon deciding that defending Mulgore was no longer enough to sate his thirst for vengeance.

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Short version is Baine exiled Tauren, including his longtime supporters, for fighting the Alliance, whilst the Alliance was trying to invade Mulgore and Dwarves were conducting a genocidal campaign against tauren.

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Looking at my initial answer, I think I would like to change it.

Have a piano fall on her head.

"But Kyalin, what if she survi-"

A saronite piano

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My old prediction was based on what we knew before 8.0 and imagining a very different story from MoP.

I will repost here my current predictions (based on what we knowe of 8.2), which I had posted in the main BfA thread a month or so ago.

Sylvanas' motivations
- Helya was the one who weakened Vol’jin, leading to his fatal wound.
- Vol’jin was already dead when Sylvanas was appointed Warchief. It was Helya manipulating Vol’jin’s corpse. That’s why Vol’jin can’t remember things clearly.
- Some other entity brought Vol’jin’s spirit back from the Other Side. My bet is Rezan.
- Once Sylvanas was made warchief, Helya leaked information about Eyir and herself to her, leading her to Stormheim and their pact.
- Helya survived her own death due to the pact. She’s now in contact with Sylvanas.
- Sylvanas’ endgame, guided by Helya, is to kill Azeroth and everyone in it, turning the whole world into a realm of undeath, with Helya as its goddess.

Reasons for the War
- The whole war is just a distraction while Sylvanas and Helya work in the background towards their endgame.
- The Void fears Sylvanas because she would lead to the death of the World Soul, and her plans would kill even N’Zoth.
- However, N’Zoth has his own plans, and is trying to get ahead of Sylvanas.
- N’Zoth made a pact with Xal’atath because he knew Sylvanas/Helya would use the blade in their plans. So, he freed the entity in it and replaced it with his own essence, essentially derailing Sylvanas/Helya’s plans.

Final Patch's possible plot
- Final patch is going to be a four-way conflict between Alliance/Horde/Sylvanas/N’Zoth. The Alliance/Horde part will be developed in questing, while the battle between Death and Void will be the theme of the final raid.
- “On the eve of her third death” is about Helya. Her death will lead to N’Zoth freeing himself.
- I still think the final patch will be set in Quel’thalas and both Helya and N’Zoth want the Sunwell to power their efforts.
- I also still think the void elves will be revealed as mostly traitors serving the Void in the final patch. Not all of them, but I think Umbric will be one of the traitors. They will lay a trap to transform more elves, since a lot of thalassians in both sides will be present in a Quel’thalas story. You know where this goes... That’s how void elves will get a “real race” status, but the "new" void elves have a completely different mindset from their predecessors, as dealing with the void was never a choice they made, so they view it as a curse.
- So, in essence, Alliance has been used as pawns of the Void ("The boy king serves at the master's table"), while Horde was manipulated by Death.
- N’Zoth will be freed but not as we imagine it. Rather than breaking his prison, his essence in Xal’atatah will gain a mortal form, in a Nyarlathotep way, and he will be set up as a villain to plague us over future expansions.

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- Sorry for any typos; english's not my primary language.

- A better signature coming soon(ish).

I like it, except for my man Umbric going evil. Don't you dare touch him.

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Daelin was right.

It's not me touching him. It's what I think Blizzard has planned for him all along.

Face it: everything about the Void Elves scream "they'll screw up at some point", and Umbric is the only named void elf that matters. Any other traitor would be met with a "who? Why should I care?" by the playerbase.

If there was a cast of void elves with differing views and attitudes, Umbric could be safe. But there's none but him to betray us.

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- Sorry for any typos; english's not my primary language.

- A better signature coming soon(ish).

And thus I don't want them to betray the Alliance at all.

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Daelin was right.

There's nothing else being built for them right now.

No hints of a storyline, nothing in the background of the patches, they just appear here and there as generic NPCs, and the only story with them so far was Umbric showing how goody two-shoes and in control he is and how helpful the void elves are. You can totally trust them, there's nothing sinister around these guys.

Unless you are Horde and see what they are doing from that point of view. In that case they are either talking about feeding souls to the void, summoning void monsters to attack Horde bases, doing their best DK impressions suffocating soldiers in void or enslaving elementals like the Twilight's Hammer usually does.

Plus, 8.2 datamined NPCs include void elves in the Twilight's Hammer. So much for them being in control.

Steve Danuser said in the article about the future of nightborne and void elves: "This is a way to show both the good and the bad of the Void Elves: They’re incredibly useful, but their utility comes with immense danger, and it’s stressed that they could snap at any time."

The only thing I see foreshadowed there is that they'll snap.

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- Sorry for any typos; english's not my primary language.

- A better signature coming soon(ish).