Koyasha is a young kunoichi training in the secluded mountains of Japan. She is overly eager to prove herself a great warrior as she can kill a person in many different ways without anybody knowing. News coming from the west about another deadly assassin known as Al' Rashid who killed over hundreds of people and just as good as herself. Seeing a competition, she sets out to kill the Arabian assassin to prove her strength and abilities. During her journey, she also heard of an evil being known as Asmodeous. She also makes this being her target, if she can eliminate these evil people, she can prove her worth.
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As Koyasha defeats both Al' Rashid and Asmodeous, she obtains the Mace, but having a just and noble soul she resists the corrupting powers of the Mace and discards it as she believes that the ruling of government comes naturally and not by superpowered forces. She returns to Japan and constructs a secluded monastery where she hopes to achieve spiritual perfection.
As Koyasha fails in her journey to defeat Asmodeous, she returns to Japan and is met with harsh punishment by her master for her arrogance and inability to curb her enthusiasm while fighting the most evil of beings. Koyasha's master condemns her to cleaning the dojo and training for years before she can go on another journey to claim the Mace. Eventually, Koyasha will become one of Japan's greatest ninjas, but it would go virtually unnoticed in a world still governed by the Covenant of Seven.