Atelier of The Demon King

Demon Lord Yami, exhausted by the prolonged war, begs Hero Lux to "kill her." But the hero couldn't bring himself to kill the lonely girl. And Yami fell in love for the first time. Lux faked the Demon Lord's death and ended the war. However, their peaceful life together didn't last long, and Lux ​​died of illness. In her despair, Yami delivered his ashes to his hometown, where she found his younger brother, Ray, who bore a striking resemblance to him. With nowhere else to go, Yami hides her true identity and takes up residence in the accessory workshop run by Ray. Eventually, she realizes that the Demon Lord's magic brings about mysterious miracles for the owners of the accessories...

(Source: Kadokawa, translated)