Oguna ~ Opera Susanoh Sword of the Devil

Once there was a military power called "Amamikado" which invaded other countries and expanded its domain.
There were two princes in Amamikado.
The younger prince was named Oguna. His father, the Emperor of Amamikado, trained him as an assassin. Oguna was hungry for power to help his father. This is a tale of an assassin before he meets Izumo.
(Paraphrased from the cover).