Hollowjack Spectre Mask is an appearance collection for the head in Elder Scrolls Online. It's part of the Holiday Event "The Witches Festival", and was added in 2017.
How to get Hollowjack Spectre Mask
Once you are undead, defeat any world bosses, delve and group dungeon / trial bosses, or dolment bosses to obtain Plunder Skulls. The mask drops randomly from them.
Masks can be traded with other players as long as they are inside a Runebox.