Oguna ~ Opera Susanoh Sword of the Devil  

Oguna - Takeru ~ Mashou no Ken Yori, oguna―takeru~SUSANOH魔性の剣より外伝

In ancient times, there existed a formidable military empire known as "Amamikado," which conquered other nations and expanded its territories. Within this powerful kingdom were two princes: Oguna, the younger of the two, and his father, the Emperor of Amamikado. The Emperor trained Oguna in the art of assassination with the intention that he would use it to aid him in maintaining control over their empire. Driven by a thirst for power, Oguna eagerly sought ways to help his father. This is the story of an assassin before he encountered Izumo.