The Holy War is an important storyline element in the Rosenkreuzstilette series. It is mentioned in the opening of the first game.
Ages ago, Magi were lived in fear of persecution from the Holy Empire and Orthodox Church. In response to this, a man named Rosenkreuz rose to correct this injustice. A powerful Magus in his own right, he and his eight disciples fought a long, grand battle against the Empire in what would be known as the Holy War. Rosenkreuz lost his life in the battle, but in return, the Empire and the Church came to accept the Magi as part of their military. These Magi became known as the RKS: The Blades of the Rose-Cross.