The Impure King is an exceptionally strong Demon. It belongs to the 'decay' category of Demons.
140 years before the current story line, it is said that there was a Demon that spread a plague of miasma, decay and fever that killed over 40,000 humans. A monk named 'Fukaku' suppressed the Demon and removed its eyes as proof of having subdued the Impure King. To this day the eyes continue to give off a strong 'Miasma' (Demonic energy) so even now they are extremely dangerous. It is said that if the left eye and the right eye are together, they will spread a new, even more dangerous and noxious miasma.
LegacyEditSaburōta Tōdō, having stolen the Left Eye, manipulated Mamushi Hōjō of the Myoda sect into stealing the remaining Right Eye for him, allowing him to implant them both into the previously sealed body of the Impure King at the Goma-do (Devil Conquering Shrine), reviving it in the process. But during this Tatsuma Suguro arrived and revealed his familiar Karura, a fire Demon that holds a contract with the descendants of the high priest Fukaku. Even though he was able to put up a good fight, Tatsuma was ultimately defeated by Tōdō who forcibly absorbed the Karura by physically swallowing it, physically empowering him and as a side effect, rewinds his age. While this fight was taking place, the Impure King was already growing to a massive size as it prepared to finish what it started 140 years ago.
Whether or not The Impure King is being controlled, or capable of acting from free will has yet to be seen.