Yūji Sakai's Reiji Maigo

A piece of this Treasure Tool was placed in a ring by Yūji to help Shana at midnight and keep her away from bad dreams... The changing color is still a Mystery but legend says that it glows from light to dark blue for each different thought... Daisuki desu !

The Reiji Maigo (零時迷子 Reiji Maigo, Midnight Lost Child) is the Treasure Tool stored inside Yūji's body. Every day at midnight it replenishes the keeper's Power of Existence up to its maximum but only so long as the bearer has not exhausted their Power of Existence.
The Reiji Maigo's appearance is of a sphere of Power of Existence surrounded by gears. It also has a Keeper (戒禁 Kaikin) that is placed by the creator in order to protect the Reiji Maigo from Bal Masqué and possible enemies who want to get their hands on it.
The name of the Reiji Maigo literally means "Midnight Lost Child". It was created by Johann, who crafted the Treasure Tool with the help of the Crimson Lord Pheles to grant eternal life to himself. Also, due to this, Pheles no longer needed to consume humans as she could tap into Johann's existence without either of them losing their existence. Unfortunately, their encounter with Sabrac forced Pheles to seal her lover, who was then fatally wounded, into the Reiji Maigo. Pheles was unaware though that Sabrac has placed the Silver inside the Reiji Maigo as well.
At the present, it was targeted by the Crimson Denizen organization Bal Masqué, with aims to concentrate an immense amount of Power of Existence by synchronizing of Bal Masqué's leaders Hecate with the Reiji Maigo. Since Hecate is known by Bal Masqué to have an "almost infinite" amount of Power of Existence, the procedure forced the Reiji Maigo to "fill something which cannot be filled", and as a consequence, it gave its current bearer almost the same limit of Power of Existence, which later becomes of help to Yūji when Bal Masqué returns to use the Reiji Maigo to give life to the Statue of Pride. It was later used to give power to the Snake of the Festival to finish the creation of Xanadu, creating a pathway to the realm between two worlds, where Xanadu will be placed.