The Grim Reaper and an Argent Cavalier

Overview

In order to attain immortality, the Grim Reaper began devouring the souls of the people. To gather more souls, he created Larvae and unleashed them upon the world. Cyan is a member of the Argent Cavaliers, an order of holy knights founded by the Grim Reaper's sister, Lemuria, that's dedicated to eradicating the Larvae. After meeting Jade, a descendent of the Reaper who is himself one of the Larvae, Cyan decides to leave the Cavaliers and join Jade on his hunt for the Grim Reaper, putting everything - including his humanity - on the line to save Lemuria, to whom he has pledged himself since childhood.

Source: AniList