Ōken

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

Overview
The only one in Soul Society that knows 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-Commander, and there is no document explaining where it is located.

Sōsuke Aizen, however, discovered that 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.