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.

The Ōken is actually revealed to be the bones of the Royal Guard. The "King's Key" is created with the power of the Spirit King, which he uses only on those chosen to be part of the Royal Guard.