The hidden Unholy Death Knight artifact skin, Bone Reaper, has been found! You can find full details, as well as handy macros, to help unlock the Unholy Death Knight artifact skins in my guide:
“How to collect all the Unholy Death Knight Artifact skins”
The TL;DR version, it appears as though everyone that has unlocked the skin has completed a hidden tracking achievement that tracks the number of Ghouls summoned via Apocalypse and has reached Artifact Knowledge level 6. There’s then a chance for you to spawn ‘Putricide’s Lost Journal’ during combat. You then travel to Putricide’s lab in Icecrown Citadel and access a hidden chamber. Kill an Abomination. Collect the skin. Profit.
For the full guide check out: “How to collect all the Unholy Death Knight Artifact skins“.