This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The historical origin of 祄 is uncertain. It combines 示 (altar, spirit) with 介, but the exact development is unclear.
The left side 示 suggests a spiritual or ritual context, and the right side 介 can mean 'mediate' or 'help'. Picture a ritual where a mediator helps connect with the divine, but the exact meaning is unclear.
For カイ, imagine a mediator named Kai helping at a shrine: Kai -> カイ, and the 示 radical sets the spiritual scene.