Ochitekita Ryuuou to Horobiyuku Majo no Kuni (Light Novel)

In a world where witches are shunned and have been forced onto the brink of annihilation, a samurai with no memories is transported to the hideout of some of these witches. Having the name: "Dragon King" and a keen strategic mind, its up to him to help defend the witches from those that would seek them harm.