Oukoku e Tsuzuku Michi (Light Novel)

This is the story of a young slave gladiator in an underground arena. He doesn't know about his past or how he get there, only that his name is Eigil, and that he is strong. One day, he kills the boss and escapes outside, joining a band of mercenaries as a new recruit. During one mission, they encounter the vampire, Lucy, who slaughters the band with her inhuman strength. After learning that Eigil only knows how to kill, Lucy lets him stay at her house, educating and take care of him. Two years pass, and on the day of Eigil's departure, they both exchange a promise that if Eigil becomes a king and owns the land of Erg forest, he can come and take her as his woman. Making this his life goal, Eigil set out on a journey to become a hero, a king, and found his own kingdom.