The Eternal Aseria

Yuto Takamine and his younger sister-in-law Kaori, were at their parents' grave, when they were suddenly summoned into another world, Phantasmagoria. The King and a woman named Restina took Kaori as a hostage, and ordered Yuto to fight against their enemies. Then, the King gave him a sword called Motomeh to protect their kingdom from enemy spirits. Kaori will only be freed if Yuto helps protect Phantasmagoria. Thus, Yuto's journey begins...

(Source: Anime News Network)