The Blood Oath's Seal is a blood pact that chains the lives of the Royal Guard together in order to leash their overwhelming power. Because a single member unleashed could shake the Three Worlds, the seal holds their true strength in check, and it can only be broken at a terrible cost.
The Blood Oath's Seal is a special blood pact shared among the Royal Guard, binding the members' lives to one another so their swords cannot reach full strength. Each of them commands so much spiritual power that one guard cutting loose without restraint could throw the Three Worlds into chaos, so the arrangement exists to keep that force pinned down.
Kirio Hikifune, Senjumaru Shutara, Tenjirō Kirinji, and Ōetsu Nimaiya serve as one another's living locks, a bond that also denies them the use of Bankai. Undoing the seal is brutal: three of the members must end their own lives so the survivor can wield their full power. By Nimaiya's account, while the pact holds, all four together fall far short of what a single one of them can manage once released.
The seal binds the Royal Guard, the elite protectors of the Soul King. Ichibe'e Hyōsube is the lone exception, since his command over his own Reiatsu is so complete that even Yhwach could sense nothing from him or from his Zanpakutō, Ichimonji, even while its Shin'uchi was active.

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The Blood Oath's Seal is a special blood pact shared among the Royal Guard that binds the members' lives to one another so their swords cannot reach full strength. It exists because each guard commands so much spiritual power that one of them cutting loose could throw the Three Worlds into chaos.
The Blood Oath's Seal binds Kirio Hikifune, Senjumaru Shutara, Tenjiro Kirinji, and Oetsu Nimaiya, who serve as one another's living locks. The bond also denies all of them the use of Bankai.
The Royal Guard uses the Blood Oath's Seal to keep their overwhelming power pinned down. Each member commands so much spiritual power that a single one of them cutting loose without restraint could throw the Three Worlds into chaos.
Undoing the Blood Oath's Seal is brutal: three of the members must end their own lives so the survivor can wield their full power. By Nimaiya's account, while the pact holds, all four together fall far short of what a single one of them can manage once released.
Ichibe'e Hyosube is the lone exception to the Blood Oath's Seal. His command over his own Reiatsu is so complete that even Yhwach could sense nothing from him or from his Zanpakuto, Ichimonji, even while its Shin'uchi was active.
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