Kyoushu Ichizoku no Suemusuko wa Godai Maou no Chikara o Yado Shite Kaikii suru

The Valgred family is an infamous clan of assassins, feared by all as the grim reapers of the battlefield. Born as their youngest son, Glenn was too inherently sickly to even train. Branded a useless failure, he is ruthlessly exiled by his own father and left to die in a freezing wasteland. At the absolute brink of death, he is saved by Emperor Wilfoen Thorngrave. To repay this life-saving debt, Glenn devotes his absolute loyalty to the empire, eventually slaying the "Five Great Demon Kings" and becoming the hero who wins the Demon War. However, his only reward is a ruthless betrayal by the very emperor he trusted. Brutally slaughtered by nine mysterious assassins, Glenn meets a tragic end. But when he opens his eyes... he finds he has inexplicably regressed thirty years into the past. Armed with the ultimate combat skills and knowledge forged in his first life, and now miraculously harboring the "Power of the Five Great Demon Kings" within his body, Glenn begins to awaken at an unprecedented, terrifying speed! Original Webtoon