Dainana Maouji Jilbagias no Maou Keikokuki

The hero Alexander and his allies launch a bold attack on the Demon King’s castle, hoping to slay him and end the war against darkness in a single decisive strike. But their plan fails spectacularly when the Demon King effortlessly crushes them. Instead of meeting his end, Alexander is reborn two years later—as the Demon King’s own son. Retaining his past memories, he resolves to use this second life to keep fighting, now as the Demon Prince Zilbagias, working to dismantle the demonic kingdom from the inside. Yet blending in as a young prince while hiding his true identity proves far more difficult than he ever expected. (J-Novel Club)