... In Quel'Thalas?
Magister Umbric and his followers were exiled because they were deemed a threat to the Sunwell. Since they wield the Void, this means that conflict arose only
after the Sunwell was purified by the Light at the end of TBC.
Which means that, in Years 20-26, Umbric and his followers were in Quel'Thalas, studying the Void and doing their duty.
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Also, I still can't believe some people genuinely wanted playable Broken instead of Void elves. Like, look at this trash:
Sorry guys, but people want to play beautiful humanoid races, not ugly monsters. It's like that in 2020, it was like that in 2017, it was like that in 2004, and it will remain like that in 2030. That's why they went for Blood elves instead of Goblin or Ogres in TBC. Void elves were the right decision.