Kiyotada Sumaru, according to the local in-game legend, is the mythical ruler of the land that would eventually inherit his name and become Sumaru City, which he lorded over in the Period of Warring States. A brutal, despotic warlord, his tyranny was such that assassination attempt was organized against him by one of his vassals, Tatsunoshin Suou, Suou's lover Maihime Amano, and Suou's retainer Junnosuke Kuroda.
Between the three of them, they were able to incapacitate Sumaru, but Amano died when the tyrant managed to gore her with a spear. Devastated by the death, Suou ordered Kuroda to flee Sumaru Castle and burn it down as he fled.
During the events of Eternal Punishment, Sumaru's remains (his head) were found in the ruins of Sumaru Castle in Honmaru Park. He was given a semblance of life by Nyarlathotep's power and he was made the centerpiece of the New World Order, communicating with the other members by telepathy. With the unsealing of the Monado Mandala, Sumaru Castle was mystically rebuilt.
He attempted to fight the party with a potent form called Gozen provided for the occasion, but was defeated.
It's implied the legend of Sumaru, Suou and the others is a fake used to warp history with rumors.