Note that you need a score of at least 200 in the Withered Army Training to get this appearance to drop. You can't just fail instantly for a chance at it.
This item unlocks the hidden appearance called Touch of Undeath for the Blood death knight artifact, Maw of the Damned. It is looted from a big chest in the Withered scenario in Suramar.To enter the scenario, you need to have completed the main Suramar questline up to Building an Army and have at least 400 x Ancient Mana. (Additional Ancient Mana will allow you to begin the scenario with better odds.)