So, while several of the Hidden artifact skins have been found or at least the community may have a general idea of where they are, we know little to nothing about the Corrupted Ashbringer skin.
What I have found looking around datamined spells and NPCs are these:
(A multitude of Alexandros Mograine NPCs added in 7.0.1)
We know as per the blue post that it is one of the most complex artifact skins acquisitions
Like others I have searched all the relevant dungeons (Stratholme, Scarlet Halls, Scarlet Monastery), comped through most of Light's Hope and the Plaguewood (as per the comic the betrayal of Alexandros Mograine that first corrupted the Ashbringer happened within sight of Stratholme's gates, between the two Scourge structures overlooking the road). Sadly I haven't found anything yet. I am wondering if perhaps it is related to Balnazzar in some way since he is also all over Legion and the way to most skins starts in Legion content. Mortiferous (the Dreadlord in Azsuna who has different emotes when facing the Ashbringer) drops nothing. I have not yet searched Niskara. Anyone has a clue?