Visage of the First Wakener

Visage of the First Wakener is the hidden artifact appearance for demonology warlocks.

To obtain the hidden appearance, you must first be at Artifact Knowledge Rank 4 or higher. You then need to farm eredar mobs in the Broken Isles or Argus for these heads, which will always drop in the following order: Damaged Eredar Head, Deformed Eredar Head, Malformed Eredar Head, Deficient Eredar Head, and Nearly Satisfactory Eredar Head.

Visage of the First Wakener will then have a chance to drop after you've looted the previous five heads.

Source
This item is a world drop from eredar mobs in the Broken Isles.