A precious jewel, within which Kagura hibernates. She reincarnates once every 100 years, then returns to the Sphere after smiting the yōma.
Every 100 years, Kagura is born from the sphere to exterminate the Yōma. Once she has destroyed an unknown unmber of Yōma, her body withers away and her essence returns to the Reincarnation Sphere.
The Goshin Clan oversees the entire process. When the time of Kargura's reincarnation is approaching, a child born the Year of Goshin is chosen to be a priestess and guardian who watches over the sphere.
The Year of Goshin is timed so that Kagura will reincarnate after the priestess comes of age (around 15 or so). The chosen child takes the name of Naraku.
Once Kagura is reborn, it is Naraku's duty to protect and guide her so that she can collect Red Orbs and mature into her true form. Once that happens, Kagura will lose all her memories and focus only on destroying Yōma until her quota is reached and she withers away, returning to the Reincarnation Sphere.
Naraku's final duty is then to reclaim the Reincarnation Sphere and pass it on to the next member of the Goshin Clan who will watch over it.
During the events of Deep Crimson, Dōgen plots to steal the power of the Reincarnation Sphere for himself.
The sphere is only seen once during the game, in a flashback to Naraku's youth. It appears as a blue opaque orb with the symbol in Kagura's eyes faintly visible within, and is about the size of a melon.