Master Therion is the Grandmaster and current head of the Black Lodge, a powerful syndicate of dark mages operating in Arkham City. He is the child of Nero, considered the greatest and most wicked magician in the world, and the Outer God Yog-Sothoth, granting him an extraordinary lineage of both human sorcery and cosmic otherness. Master Therion possesses a youthful appearance with medium-length blonde hair and yellow eyes, and he carries himself with an expression of contemptuous boredom. His personality is defined by a profound sense of ennui; he finds little interest in those who cannot match his immense mystical power, and he derives his only genuine delight from causing acts of evil and cruelty, which briefly ignite a fire in his otherwise languid eyes. His primary motivation is the alleviation of boredom, which shapes his antagonistic role. Rather than destroying his enemies outright, he often allows them to survive if they provide him with amusement or a worthy challenge.

In the story, Master Therion serves as the main antagonist. He leads the Black Lodge in its quest to reshape the world and control the Necronomicon, the most powerful grimoire. His first direct confrontation with the protagonist Kurou Daijuuji occurs shortly after Kurou acquires the Necronomicon and pilots the Deus Machina Demonbane. Master Therion easily overpowers Kurou, revealing his own grimoire, the Pnakotic Manuscripts known as Etheldreda, but spares Kurou's life, claiming that Kurou is the only individual capable of alleviating his boredom. This sets the pattern for their repeated clashes throughout the series. Key relationships include his rivalry with Kurou and his command over Black Lodge members such as the mad scientist Doctor West. He also controls the Deus Machina Liber Legis, a god-machine that matches Demonbane in power.

Character development appears in certain narrative paths. In the Al Azif route of the original visual novel, Master Therion is eventually freed from an eternal cycle known as the Infinite Spiral by Kurou and thereafter watches over humanity from a distance. In the sequel series Kishin Hishou Demonbane, he is reincarnated in a different world under the name Perdurabo, becoming a benevolent figure and forming an unusual friendship with a new protagonist. His notable abilities include vast mystical power that surpasses almost all other sorcerers, powerful telepathy capable of communicating with everyone in Arkham City simultaneously and attacking the minds of his foes, and mastery over the Pnakotic Manuscripts and the Liber Legis. He is considered so formidable that only the perfected combination of Kurou, Al-Azif, and the Demonbane mech can truly stand against him. His combination of charisma, ruthlessness, and overwhelming might makes him a central figure in the conflict between the Black Lodge and the Hadou Financial Group.