This headwear is cursed. A creature that attunes to it that is not a worshipper of Myrkul becomes a beacon to any Grey Ones of Myrkul that are on the same plane, and to Myrkul himself no matter the plane, and the mask immediately attempts to bite their face (DC 18, If the Finger of Death kills them, their zombie will tirelessly travel to deliver the headwear to the closest Grey One). The cursed creature has disadvantage on concealing themselves from undead and on death saves. Once the headwear is donned, it can't be removed unless the wearer is targeted by the remove curse spell or similar magic.
Note on Cursed Items: (DMG 138)
Cursed Items: Most methods of identifying items, including the Identify spell, fail to reveal such a curse, although lore might hint at it. A curse should be a surprise to the items user when the curse's effects are revealed.
If it requires attunement, you are not only cursed as long as that attunement exists, but you are prevented from ending the attunement on your own voluntarily.
Remove Curse, or Greater Restoration will end attunement to a cursed item long enough for it to be removed, and the character freed from that curse, but will NOT end the curse on the item.
The only methods to remove a curse on an item typically is the use of a Wish, destroying the item entirely, or in the case of a deliberate curse being cast during creation, the caster must release it.