The Ashen Slayer

Several years after the destruction of the Agarthans and the defeat of Nemesis and his undead Ten Elites, Byleth chose to leave behind the duties and responsibilities of Garreg Mach Monastery, instead choosing to return to the life of a mercenary and travel the world. But one last remaining Agarthan somehow managed to survive and as revenge before its death he used a teleport magic with its last strenght and Byleth soon found herself at the country of Japan during the Taisho Era. There she would resume her work as a mercenary, however, she would soon come to realize an even greater threat than she could have realized, hiding in the shadows... The demon race... Led by their ruler, Muzan Kibutsuji. Following a chance encounter with a strange young man with bright fiery yellow and orange hair, Byleth was soon accepted into the ranks of the Demon Slayer Corps, vowing to aid them in their plan to defeat Muzan Kibutsuji and end the demon race, once and for all...

A few other Fire Emblem characters are there as well
Thanks on GreninJoker for making this awesome Cover
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Demon Slayer is from Koyoharu Gotouge/Shūeisha/Ufotable
Fire Emblem is from Nintendo