Ōken

Presented as a key granting passage to the Soul King Palace, the artifact is in truth the skeletal remains of the Royal Guard, transformed by the Soul King's power the moment a member is chosen for Division Zero. Because the key is literally part of their bodies, only two routes into the palace exist: to be admitted by a guardsman's own will, or to enter at his side. The single figure in Soul Society aware of this secret is the Captain-Commander of the Gotei 13, who hands the knowledge down by word of mouth alone.

One can be forged from a hundred thousand souls gathered at a Jūreichi, a dense spirit ground spanning a radius of one spiritual ri, the role currently filled in the living world by Karakura Town. Since the key is made of a guardsman's bones, that process effectively births a new member of the Royal Guard outright. The material has a second use as well: Senjumaru Shutara can spin the Ōken within her own body into clothing of extraordinary defense, garments able to shield a wearer against the friction of tearing through the seventy-two barriers dividing the Soul King's realm from the Seireitei, an act that leaves those barriers unable to seal for one hour and forty minutes. Durable as it is, such attire is not invincible, and overwhelming force or extreme cold can still ruin it.

The key is embodied in every member of the Royal Guard, the elite Division Zero created by the Soul King. Ichibe'e Hyōsube is the one who lays bare its true nature, correcting the earlier belief that it was simply an ornate key. Senjumaru Shutara turns hers into the celebrated Ōken clothing, protective gear that even Yhwach praised as the finest a Shinigami could wear.