Ōken

An is a "key" that allows passage to the Soul King Palace.

Overview
An Ōken is the bones of the Royal Guard, modified by the power of the Soul King. The only person in Soul Society who knows this is the Captain-Commander of the Gotei 13, who passes it down orally.

An Ōken can be forged with 100,000 souls and a with a radius of half a spirit-mile. The current Jūreichi in the Human World is Karakura Town. Since an Ōken is the bones of the Royal Guard, this method literally creates one of the Royal Guard as new life.

Senjumaru Shutara can use the Ōken in her body, which would consist of her hair and bones, to create clothing. Clothes designed in this manner possess incredible defensive powers, and can protect the wearer from the friction of breaking the 72 layers of barriers that separate the Soul King's dimension and that of the Seireitei. Should this happen, the 72 barriers cannot be closed for 1 hour and 40 minutes because of the outfit's sheer defensive power.

Appearances in Other Media
In Bleach: Can't Fear Your Own World, Ichibē Hyōsube states that due to the Ōken being embedded within their bodies, the very life essence of each Royal Guard member is linked to the Reimyaku of the Soul King Palace and therefore he is able to revive any member of the Royal Guard by calling their name as long as the palace itself continues to exist. This also seems to grant the bearers of the Ōken a form of agelessness, as Ichibē has guarded the Soul King for millions of years without growing older. Indeed, even Ōetsu Nimaiya has been alive since the youngest days of Genryūsai Shigekuni Yamamoto, yet has not notably aged as opposed to the other, elderly Shinigami.

Trivia

 * When the Ōken is first mentioned, it is represented as an actual key. This is later proven inaccurate by Ichibē Hyōsube.

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