The question I ask is, what would Helya and her Val'kyr know that could do such a thing, and what on Azeroth is wholly unique enough that it would disable the Arbiter, while uncountable souls from all worlds across all universes could not?
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We also have to take into account that the Jailer helped Sylvanas become Warchief. Any other would still have killed Gul'Dan and Argus, her promotion wasn't necessary for that.