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 idea being that the location will be 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, therefore it is literally in their bones. Aizen's attempt to forge a key for himself is therefore, to create a new life itself.

The Royal Guards can use the Ōken in their bodies, which would consist of their hair and bones, to create clothing for Shinigami. Clothes designed in this manner possesses 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 of for 1 hour and 40 minutes because of the clothes' sheer defensive powers.