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One of the most powerful onmyouji from Abe no Seimei's era. He was the rival of Abe no Seimei and fought him nearly a thousand years ago. Ashiya Douman has shown interest in Yakou's reincarnation and seems to have connection with Twin Horn Syndicate/Bicornis Fellowship and Kakugyouki. He also fought with Jin Ohtomo which resulted in Jin losing his leg.

(Source: Wikipedia)