Ōken

The  is the key of the royal house that can open a gateway to the dimension where the Spirit King is found. The only one in Soul Society that knows the location of the King's Key is the Commander-General of the Gotei 13, currently Shigekuni Yamamoto-Genryūsai; the location is passed on verbally from generation to generation of Commander-General, and there is no document containing where it is located.

Aizen, however, discovered that the King's Key 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.