Tokisada Shiro Amakusa began as a historical Japanese Christian leader during the Shimabara Rebellion, defending persecuted peasants and Christians against oppressive shogunate forces. As a youth, he survived an assassination attempt when a bullet struck a crucifix he carried, leading followers to declare him a messianic figure. Commanding rebel forces from Harano Castle, he initially advocated mercy toward enemies and inspired loyalty through spiritual leadership, displaying motivational posters throughout the fortress. His forces repelled initial shogunate attacks through coordinated defenses. However, his lieutenant Mori Soiken manipulated events by murdering two children to provoke Amakusa. Witnessing this atrocity shattered Amakusa's faith and compassion, triggering his transformation into a demonic entity. In this state, he summoned a black dragon against the swordsman Yagyu Jubei but was ultimately defeated and left for dead in the castle's burning chapel.

Mortally wounded and spiritually broken, Amakusa became consumed by Soiken's corruption. Through the Makai Tensho ritual, involving Soiken's daughter Ocho, he was reborn as a vessel of Satan. The ritual required Ocho's body to undergo a grotesque transformation during a blood moon, facilitating Amakusa's resurrection years later. Upon returning, he embraced his role as an agent of destruction, commanding the resurrected spirits of legendary warriors—Inshun Hozoin, Mataemon Araki, and Botaro Tamiya—to slaughter civilians and shogunate loyalists. His motives shifted entirely to apocalyptic vengeance against Japan, seeking to plunge the nation into chaos as retribution for the suppression of Christianity and his own execution. He led massacres during public events, signaling the start of his campaign, with the blood moon serving as an omen of his wrath.

Amakusa embodies a fall from idealistic martyrdom to nihilistic villainy. His background as a charismatic, compassionate leader contrasts with his post-resurrection identity as a cunning and merciless force. He wields supernatural abilities, including demonic transformation and command over undead warriors, to execute his vengeance. His development culminates in abandoning his messianic purpose, embracing destruction as an end in itself.