Born as an infant into the Fujiwara clan, Douman was cast out because he was marked with visible scars. Despite being abandoned to die, he survived and eventually became an apprentice to the powerful priest Douma Hoshi. In a brutal act emblematic of his ruthless nature, Douman would later murder his mentor and take the priest's title of Ashiya Douman as his own name.

Douman's primary motivation is rooted in the deep-seated hatred born from his rejection. He despises the world that abandoned him and seeks the utter destruction of the Light Palace, the stronghold of his brother, Fujiwara no Michinaga. This desire to see everything annihilated drives his actions, as he aims to plunge the world into darkness. He defines himself as darkness itself, claiming that only darkness remains in his heart. Unlike the Horrors he commands, Douman is fundamentally human, though he is described as a cursed being whose soul is thoroughly steeped in evil.

In the story, Douman serves as the central antagonist and the orchestrator of the Horror invasions plaguing the capital. He manipulates the downtrodden and the spiteful, exploiting their resentment against the rigid social structure to turn them into Horrors that do his bidding. His ultimate scheme is to awaken the ancient and cataclysmic Horror, Rudra, believing this act will finally bring about the destruction he craves. He is willing to go so far as to have Rudra devour him upon its revival, and later, he fuses his very being with the monstrous entity to gain even greater power.

Douman holds key relationships that define his path. He is the estranged brother of Fujiwara no Michinaga, who rules the Light Palace, and he rejects Michinaga's offer of reform and a place at his side. He is also a master manipulator, having been an apprentice to the ancient and corrupt priest Douma Hoshi, whose schemes he continues. As a dark onmyouji, he stands as the direct archenemy of the Makai Alchemist Abe no Seimei, considering her and all other practitioners from the Light Palace to be beneath him.

Throughout the narrative, Douman's actions only grow more ambitious and devastating. He consistently works to unleash chaos, from freeing powerful spirits to sending his enemies into alternate dimensions and corrupting individuals into Horrors. In the final confrontation, he succeeds in resurrecting Rudra and merges with the Horror, creating a monstrous fusion that nearly overwhelms the heroes. However, after Rudra is defeated, Douman is separated from the creature. In a final duel with the protagonist Raikou, he is defeated. In his last moments, he is shown not in defiance, but in a moment of acceptance as he fades away, held by Seimei.

As a formidable onmyouji of immense power, Douman possesses a range of frightening abilities. He can produce thorny vines to ensnare his opponents and has the power to curse others, a skill he uses to cause instant death. His most grotesque ability is using his heavily scarred face as an "Inga Gate," which allows him to directly infect people with Horror possession. He can also create portals to summon Horrors, teleport, and send enemies into a pocket reality under his control. When fused with the Horror Rudra, his power expands exponentially, granting him massive size, enhanced strength, regeneration, and the ability to manipulate the weather and devour souls.