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This is the hidden appearance for Outlaw rogues.  It is the appearance of the classic  Thunderfury, Blessed Blade of the Windseeker except, you know, one for each hand.

To get this, you need  Bindings of the Windlord from the last boss in  Neltharion's Lair and  Bindings of the Windlord from the Golem boss in  Vault of the Wardens.  These items can drop on any difficulty, including normal.  The drop rate was raised in patch 7.2 so it shouldn't require too many runs to get them now.

Once you get both bindings, obtain x10  Infernal Brimstone (you will need this for the final stage of the quest) and then head to Vethir in Stormheim.  For Alliance, you can find him at Lorna's Watch and for Horde, he will be at Cullen's Post.  Do note that you need to have done enough of the Stormheim quest chain to have "unlocked" his presence.  Talk to Vethir and he will give you several dialogue options. Select the one that sends you to the top of Thorim's Peak.

Once up there, you need to speak to Thrymjaris.  You must be in Outlaw spec for her to offer the correct dialogue options.  The second choice will reference a Windlord.  That's the one you want.

Select it and and Thunderaan should appear and have a bit of dialogue and then offer Audience with the Windlord.  Complete that and receive your hidden appearance!

This item unlocks the hidden appearance called did somebody say Thunderfury, Hallowed Blade of the Windlord for the Outlaw rogue artifact,  The Dreadblades.

There is little information on how to obtain it, though it seems RNG will play a large part.  A Chinese player looted the left half  Bindings of the Windlord from Dargrul on Mythic-difficulty  Neltharion's Lair.  The source of the right half  Bindings of the Windlord remains unknown.  There is probably a quest that exists for turning in the items.