From Ash and Blood

Years after Arthur takes the throne, a rebellion takes hold in the former kingdom of Dunbray. Tristan and Lancelot, newly-made Knights of the Round Table, work to uncover the people behind it, all the while questioning if Camelot is in the right; when they call on Mael to help, he winds up far more entangled in the rebellion's affairs than he ever wanted to be, drawn by both their plight and the one who now wields the sun.

Set after the ending of Nanatsu no Taizai, but unrelated to its sequel.