Ōken

The is the key of the royal house which allows passage to the dimension where the Soul King resides.

Overview
The precedent is that the only one in Soul Society which would have the location of the Ōken is the Captain-Commander of the Gotei 13. The location is passed on verbally from generation to generation of Captain-Commanders, and there is no document explaining where it is located.

Sōsuke Aizen, however, discovered the 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.

The Ōken is in reality the power given to those chosen to become Royal Guard. The Soul King manifests the power into those individuals, so it is literally in their bones. Therefore, Aizen's attempt to forge a key for himself was an attempt to create a new life.

Senjumaru Shutara can use the Ōken in her body, which would consist of her hair and bones, to create clothing for Shinigami. 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 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.