This is Erebus with a Dark Mask on. Its the same kind the Hunters wear, only it fits him. He wears it for protection against his rivals, and can spawn it whenever he wants to.
The mask he wears completely protects his face. The only thing exposed is his eyes.
EDIT: I decided to mask his eyes not visible when he wears his helmet. It looks better in my opinion, and it also makes him look more like hes wearing a helmet, and his eyes are not visible due to the shadows.