The Inboyuigon (最終的な証拠; lit. Final Testament) were a cult of spiritual beings of great power. Residing in secrecy within Kongoukai, they were once dangerous enough to warrant the full attention of Soul Society, though they were temporarily 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. Per their history with the Seireitei, and information gained from higher sources, they also regained their knowledge of crafting spiritual weapons on par with the Zanpakutō, and techniques similar to the Kidō arts.
The Inboyuigon met their ultimate defeat from the most unlikely source, one of their own leaders. After the ascension of the Sasageru Raishuu Onmyou, the newly born Seimeigami executed the Ichijou-Banji, which threw the Undouka into sheer chaos, resulting in their quick descent and collapse.