Following the Hollowfication Incident which took place in Soul Society over a century prior to the start of Bleach's main storyline, there emerged a certain group of Lieutenant and Captain-level Shinigami who were able to manifest Hollow powers — effectively the antithesis of the Arrancar in Bleach. This was a consequence of the ordeal they had suffered due to Sosuke Aizen's experiments. However, on finding and retrieving them from the scene of the crime, former 12th Division Captain Kisuke Urahara was able to stabilize their condition with an object he created known as the Hogyoku.