Ranmaru Mori, a primary antagonist, is an extraterrestrial entity who arrived on Earth via a meteor impact in 946 AD alongside the demonic warrior host, the Oboro. His true objective involves manipulating history to open a gateway to the underworld by harnessing the negative emotions generated through prolonged conflict. He orchestrates a centuries-long scheme, presenting himself as a loyal advisor to Oda Nobunaga while secretly controlling him.

Granting Nobunaga command over the Oboro and supernatural powers, Ranmaru promises the conquest of Japan, though his actual intention is to use Nobunaga as a vessel to amplify human suffering and consolidate power. He engineers the prophecy of three mystical weapons—distributed to the ninja clans of Kasumi, Hyuga, and Hagakure—knowing their wielders would inadvertently fuel his plans through their battles. Throughout the conflict, Ranmaru maintains a facade of subservience, exploiting Nobunaga's paranoia to eliminate threats.

His manipulation extends to deceiving the protagonists—Ayame, Sakon, and Ryoma—by providing their clans' enchanted blades centuries earlier, ensuring their quest for vengeance against Nobunaga would generate the required emotional energy. During the Honnōji Incident, Ranmaru substitutes duplicates to conceal his and Nobunaga's survival. He later accelerates his endgame during a celestial alignment, attempting to merge the Oboro's power with accumulated negative energy to transform Nobunaga into the Black Demon, a vessel to open the underworld gate.

Ranmaru's plans unravel when the protagonists discover his role as the architect of their suffering. In the final confrontation at Azuchi Castle, Ayame, Sakon, and Ryoma use their combined weapons to disrupt his ritual. Sakon and Ryoma sacrifice their lives to weaken him, enabling Ayame to deliver the killing blow. Ranmaru is destroyed, his ambitions thwarted, leaving Ayame as the sole survivor. His legacy as a manipulative, otherworldly instigator defines the conflict's supernatural dimensions.