Tokin

A Tokin(頭襟, ときん) is a small black box worn on the foreheads of Yamabushi - practitioners of Shugendō - or Tengu, dangerous yet protective spirits of the mountains and forests from the Japanese mythology.

It is not only worn as a head decoration, but also used as a drinking cup.
 
"Tokin" is also the name of a "promoted pawn"(と金 in Japanese) in shogi (Japanese chess).