Demon mask

is a type of mask that made up of demon bones, it is used for controlling victim as he wears on.

During the Heian jidai, Lord Gorai who came from the Northern Lands ruined the wedding ceremony between Datara and Tsugumi. He used his daughter's life to forced Datara wore on the demon mask, making him turned into the God of Demon. But as soon as Datara wore it, Tsugumi used the Arrow of Sealing to seal his husband for preventing negative possibilities.

500 years later, Gorai sent Monk Sen to lure Inuyasha and the others to undone the seal of Datara at the Datara Shrine. Although Janis successfully removed the arrow from Datara's chest, Gorai still did not have any idea to get Datara's Kamuitama, since it was too hard to control by a mere yōkai like him. After Inuyasha and the others defeated Gorai and Datara at the Tempest Tower, Janis returned her Kamuitama to Datara, the demon mask slowly slipped down from his face and broke into pieces. Datara was no longer being controlled.

Reference
妖怪面具