Twelve Kitaiyō

The Twelve Kitaiyō is a band of fourteen powerful demons, which are the fallen Hashiras and Demon Slayers.

Overview
After all of the Hashiras and Demon Slayers were converted, they were taken to the demon realm and forced to serve Mujōna. train in order to become stronger. Most of them are unable to retain memories of their lives as humans, or have completely disregarded their pasts.