Blood Oath's Seal

The is a special blood pact that prevents Royal Guard members from using the full power of their swords, by linking their lives together.

Overview
Because each individual member of the Royal Guard possesses vast levels of Reiryoku, they can destabilize the three worlds by themselves should they unleash it all without restraint. Therefore, Kirio Hikifune, Senjumaru Shutara, Tenjirō Kirinji, and Ōetsu Nimaiya normally use each other's lives as seals to prevent their true power from being released; this also prevents them from being able to use their Bankai. To unlock this seal, three of the members have to take their own lives in order to enable the lone survivor to use their full power.