Dōman Ashiya is the primary antagonist of the anime Garo: Crimson Moon. He is a mysterious and powerful onmyōji, a master of mystical arts, who identifies himself with darkness and chaos. His name is not his birth name but a title inherited from his mentor, the priest Dōma, whom he eventually murders. His face is notably marked by numerous scars, a physical manifestation of his past rejection and a key component of his dark abilities.

Dōman was born into the powerful Fujiwara clan, making him the brother of the influential nobleman Fujiwara no Michinaga. As an infant, he was cast out by his clan because he was born with these disfiguring scars. This rejection instilled in him a deep-seated hatred for the world, particularly the light and the established social order that cast him aside. His entire personality is forged from this resentment; he is described as a human whose heart is completely tinged with evil, leaving room only for darkness. He is manipulative, cruel, and takes pleasure in exploiting the same feelings of bitterness and exclusion in others to turn them into Horrors or use them for his schemes.

Dōman's primary motivation is to destroy the capital of Heian-kyō and everything it represents, especially the Light Palace ruled by his brother. He seeks to awaken the ancient and powerful Horror named Rudra. His ultimate goal is to merge with this entity, effectively plunging the world into an age of darkness as a final act of revenge against the society that abandoned him.

In the story, Dōman acts as a hidden puppeteer, constantly working behind the scenes to destabilize the capital. He frequently appears before the protagonists, including the Golden Knight Raikō and the alchemist Seimei, to taunt them or set his plans in motion. His schemes include turning disgruntled nobles into Horrors, releasing sealed monsters like Itsumaden, and manipulating political figures to increase suffering. Much of his effort is directed at breaking the seals that bind Rudra while also trying to isolate Raikō from Seimei, whose magic is essential for the knight to summon his armor. Dōman is also revealed to be the one who transformed Junkei into a Horror at the series' start, cementing his role as the architect of the chaos that plagues Heian-kyō.

His key relationships are defined by opposition and twisted family ties. His antagonism toward his brother Michinaga drives much of his destructive behavior, showing a desire to tear down what his sibling protects. He has a deep, hateful connection to the protagonists, especially Raikō and Seimei, viewing them as representatives of the light he despises. While he was the disciple of Priest Dōma, this relationship ended in murder, with Dōman killing his master and taking his name.

Throughout the series, Dōman remains a relentless and seemingly unstoppable force. However, his development culminates in the final battle when he successfully resurrects Rudra and fuses with the Horror. This act, meant to make him the supreme agent of darkness, ultimately leads to his downfall. After Raikō defeats Rudra, Dōman is separated from the Horror and engages in a final duel with the Golden Knight. In a moment of redemption, Dōman accepts the light just before he fades away, dying in Seimei's arms.

In terms of abilities, Dōman is an exceptionally powerful sorcerer whose skills rival or even surpass those of Seimei. He can summon thorny vines to ensnare his enemies and has the power to create Horrors. His most distinctive ability is tied to his scarred visage; his face acts as an "Inga Gate," a conduit through which he can infect others with Horror corruption directly. He is also skilled in creating barriers and manipulating space, capable of trapping opponents like Seimei in alternate dimensions. His magical power is such that he can withstand and counter advanced magical attacks with ease.