Every year in hell there is a cleanse, where Heaven sends down an army of angels known as exorcists to exterminate demons to stop the over population.Usually casualties were kept at an average percent until he showed up. Once he showed up the percent of casualties became brutal. It was like never before, usually the casualties consisted of sinners,imps, hell hounds, and hell borns,but now it included overlords. Only a few of them are able to get glimpse of him and live to tell the tale. Wields only a sword, a white haori, and black uniform They call him The Demon Slayer