Oukoku e Tsuzuku Michi - Dorei Kenshi no Nariagari Eiyuutan

Aegir, a young slave gladiator who knows neither his past nor how he came to be in an underground arena, knows only his name and his strength. After killing the arena’s boss, he escapes and joins a mercenary band, but their mission brings them face to face with Lucy, a vampire whose inhuman power leaves the band dead. Learning that Aegir knows only how to kill, Lucy takes him into her home and cares for and educates him. Two years later, when he is set to leave, they make a promise: if Aegir becomes a king and claims the land of Erg Forest, he may return and take her as his woman. With that goal before him, Aegir sets out to become a hero, a king, and the founder of his own kingdom.