The Northern Grand Duke’s Youngest Son is a Genius Scoundrel

All his life, Kairun had suffered under the label of being the Northern Grand Duke’s illegitimate son and longed for his father’s acknowledgment, yet in the end, he lived and died as a worthless scoundrel alongside the fall of the Empire. At the very moment before death, he attained enlightenment at the realm of a Grand Master and regressed to his childhood, when he had been subjected to human experimentation. — Description from novelcodex.org