Raise Me, Hero!Yuusha yo, Ware o Sodate yo!; 勇者よ、我を育てよ！; Yuusha yo, Ware wo Sodate yo!; Raise Me, Hero!

Demon King Velgar met a dramatic end after his battle with the hero Karen. Although the Demon King left behind his last words, "In my next life.. I swear I'll come back to destroy you!!!," he later does indeed achieve reincarnation. What awaits him in his humiliating new life after being reborn... Source: MangaPlus