Dai Nana Maouji Jilbagias no Maou Keikoku-ki

The hero Alexander and his comrades unleash a daring raid on the Demon King’s castle, intent on assassinating him and bringing an end to the war against the darkness in one fell swoop. However, the raid goes awry when his party is effortlessly decimated by the Demon King’s overwhelming power. But death is not the end of the road for Alexander, as he is reincarnated two years later as that very same Demon King’s son. With the memories of his past life still intact, he vows to use his miraculous second chance to continue fighting—this time destroying the demonic kingdom from within as the Demon Prince Zilbagias. It doesn’t take long for his mettle to be tested either, as passing as a young demon prince without blowing his cover proves to be quite a bit more challenging than he initially anticipated. (Source: J-Novel Club)