In a world resembling the middle-age. A girl, Riannon, is set to be sacrificed to appease a resurrected demon lord, Arawn. As her brother Arthur attempts to rescue her, Arawn defies those who resurrected him and frees Riannon from her captors, which leads to Riannon admiring him.
This eventually leads to a party of companions being assembled for adventure, with Arawn acting as the leader.
— Description from AniList community (anilist.co).
