Chances are, if you ever had a priest you know this icon.
Shadowform, makes you into a purple translucent shadow of yourself.
Outland had a lot of shadow priests in it, and well, the necrolytes and junk came from Draenor, do you think we could discover a thing or two about this mysterious spell every other priest has?
Would you like the green eyes the icon has? To me it always looked like fel fire so I wouldn't be too supriced it it had some roots on demonic origins