Gyanon is one of strongest Horrors in existence, labeled as Messiah's Fang (メシアの牙, Meshia no Kiba) and the Ancient Red Demon Beast (太古の赤き魔獣, Taiko no Akaki Majū).
Having seemingly died, the Makai Senate sent a group of three Makai Knights and one Makai Guide to retrieve the monster's corpse from Amber Rock Valley. But Barago also sought the Horror to consume the monster. However, both parties were beaten to the punch by Sigma Fudō who took the Horror to serve as the core in his Magōryū Idea while reviving the fiend with large numbers of human sacrifices.
Wanting to ensure control over the revived Gyanon, having to siphon some of the Horror's powers, Sigma obtains the pelt of a Spirit Beast while infusing Kouga's body and soul into the Horror to complete his resurrection. Though Kouga is freed, Gyanon awakens after Idea's creation in response to Sigma's darkness, devouring him and assimilating the ultimate Gōryū while assuming his androgynous human-like form. While telling Garo that his resurrection was unplanned, Gyanon intends to take advantage of the situation to bring all Horrors into the human world. However, Idea is destroyed with the Makai Knights and Madou Priests joining forces with Gyanon reduced to a head as Garo destroys the bodiless Horror.
- Gyanon is named after the Buddhist entity called Guanyin (Kanon in Japanese), which plays into Gyanon's androgynous appearance.