The Vessel was a shrine maiden, raised in complete isolation except for an old woman who would care for her, and sacrificed in the Tokoyomi Ritual. A spirit would be called down to possess her, and to ensure that the spirit could not escape if it happened to be evil, the maiden's eyes and mouth would be sewn shut, thus "sealing" her.
The shrine maiden would grow up alone in the Old House, to ensure her suitability as a vessel. Any worldly bonds would be like "cracks", permitting the spirit to escape.
When the shrine maiden reached the age of fifteen (eighteen in the European version), at a time when the village was in need of divine intervention, the Seamstress would seal her eyes and mouth, and perform the rite of greeting, which would summon a spirit to dwell in her. There was no way to predict whether the spirit would be good or evil. A good spirit would inhabit the maiden and cause the village to prosper; an evil spirit would devour her soul and fill her with darkness.