18+ Oukoku e Tsuzuku Michi

This is the story of a young slave gladiator in an underground arena. He doesn't know about his past or how he got to where he is, only that his name is Aegir, and that he is strong.

One day he kills the boss and escapes the arena, 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 Aegir only knows how to kill, Lucy lets him stay at her house, educating and taking care of him.

Two years pass, and on the day of Aegir's departure, they both exchange a promise that if Aegir becomes a king and owns the land of Erg forest, he can come and take her as his woman. Making this his life goal, Aegir set out on a journey to become a hero, a king, and found his own kingdom.

(Source: Novel Updates)