Mark of the Shitōshin

Seen in the fourth Inuyasha movie. The mark is obtained if one of the Four War Gods(Shitōshin) scratches ones back. It causes one immense pain when glowing. When the mark is glowing, it means fifty years have passed, and it is time for a new sacrifice. If one possesses the mark, and goes to Horai Island, they will be unable to leave. Each mark represents a god, and each time a god is destroyed, a mark dissapears. When all four marks have vanished, and all four gods are dead, the person who once bore the mark is now free to go.