Inboyuigon

The Inboyuigon (最終的な証拠; lit. Final Testament) were a cult of spiritual beings of great power. They were once dangerous enough to warrant the full attention of Soul Society, though they were defeated during a cold war with the Shinigami. The Inboyuigon were considered strong due to their unorthodox methods of gaining power. Interbreeding of individuals with high reiatsu was their way of creating strong warriors over long periods of time. Through infiltration of the Seireitei, they also regained their knowledge of crafting spiritual weapons on par with the Zanpakutō, and techniques similar to the Kidō arts.