Arms Kabuki

Eight-Headed Serpent, the leader of these monsters, was, to be precise, one of the few bosses who had never revealed its true form in the TV series.

If Gu Yu had not read Kamen Rider Hibiki's SIC novel and known the little story from it, he truly would not have been able to identify this guy's true identity after merely a few dozen seconds of fighting.

In the SIC Hero Legends side story, the Oni-corrupted monster Futakuchi-onna transformed into Dance-Song Demon and fought the Seven Warring Demons of that era.

Afterward, Eight-Headed Serpent, which had split its body into eight, sent all of its bodies to attack the seven warrior Oni present.

Since one of the Serpent's clones had already been slain by an Oni, it could not create another body to establish an advantage through numbers.

Hibiki seized that opportunity and unleashed the [Kusanagi Sword] of that world—the long sword now held by Great Serpent Dance-Song Demon, the [Musha Sword]. He evolved into Armor Hibiki mode and led everyone in destroying six of the Serpent's bodies in one stroke.

Seeing that its final body could not defeat them, the Serpent swallowed Futakuchi-onna, who had transformed into Dance-Song Demon, whole. After using her as a sacrifice to restore its strength, it fled the battlefield.

Because of this incident, Hibiki and the others of the Warring States era chose to establish an organization called [Musha] afterward, supporting the Oni in their continued battles.

Considering that the previous bosses had all been dead people, perhaps the Serpent had also been destroyed in the future world.

Yet at the same time, considering how all those unlucky bastards who had been defeated had revived in this otherworld at full health...

—The current Serpent had already regained its eight heads and could split into clones