Posing as a loyal retainer to warlord Oda Nobunaga, the demonic advisor Ranmaru secretly manipulates events to further his own agenda. He arrived on Earth in 946 AD as a fragment of an evil star or comet, waiting centuries to enact his plan for dominion. Ranmaru feeds Nobunaga's paranoia and guides his conquests, promising power while secretly controlling the Oboro clan's shadow warriors and demonic forces. Unbeknownst to Nobunaga, Ranmaru himself is the most powerful Oboro entity, the Black Demon.

Ranmaru orchestrated the prophecy of the three mystical Demon Blades, deliberately distributing these weapons to ninja clans centuries earlier. He manipulates both Nobunaga and the blade-wielding protagonists—Ayame, Sakon, and Ryoma—to gather negative human emotions from the conflict. His goal is to harness this energy combined with the Oboro's power to manifest his Black Demon form. His machinations include foretelling the blades as Nobunaga's only weakness, ensuring continuous warfare to fuel his scheme.

During the climax at Azuchi Castle, Ranmaru's true objectives are revealed. He attempts to open a gate to the underworld as the evil star approaches Earth closest, seeking to merge its power with his own. After Nobunaga's body double is killed during Akechi Mitsuhide's betrayal at Honnō-ji, Ranmaru prepares to transform Nobunaga into the Black Demon vessel. The three protagonists confront him, with Ayame, Sakon, and Ryoma combining their weapons' powers to defeat him. Ranmaru is ultimately killed in this battle, though Sakon and Ryoma sacrifice their lives.

His influence extends to corrupting allies like the monk Ryoan, an unwitting Oboro agent who aids the protagonists before Ranmaru impales him. Ranmaru's mastery of manipulation ensures all parties, including Nobunaga and the heroes, remain pawns until the final confrontation exposes his plan.