Aoki Shichiro was a fake cultivator whose power was divine. However, there was not a single King of Temporal Conquest temple in the Eastern Land to transmit divine power. On Aoki Shichiro's right thumb, he wore a special ring with a symbol of an inverted cross within a circle engraved on it. This ring was a "container" that stored a massive amount of divine power from the evil god.
It was the custom of the Red Barbarians to wear rings, and this "container" divine artifact was originally in the form of a ring. The opponents on Sang Island discussed and decided to turn it into a thumb ring since men in the Eastern Land generally didn't wear rings. The design was a bit peculiar since it was modified in haste.
Aoki Shichiro's challenges were similar to dojo storming, where he would find well-known competitors and send them direct challenges to their doorstep. No matter if the opponents accept or not, the fight would start right away.