Ōken

The is the key of the royal house which can open a gateway to the dimension where the Soul King is found.

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 plot of spirit-enriched land with a radius of half a spirit-mile. The current spirit-enriched land in the Real World is Karakura Town.

The Ōken is revealed to be 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.