Wowhead says that a spell named "shadow walk" is granted by illidan at the beginning of the well of eternity 5man. Not sure if I'm right but wasnt that instance about demons?
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Well actually I don't think its the well of eternity thing.
But I found this:
https://www.youtube.com/watch?v=lvwR-uELdS4
Some guy saying there's a puzzle and maybe a hidden boss in Vault of Wardens.