The Grand Duke's Son Is A Heretic Novel

Leonard, the world's greatest heretic, was betrayed by everyone—his sister, humanity, and the gods themselves. After accepting a demonic contract to spy on demon forces during their invasion, he sacrificed part of his soul to save humanity. But the war was merely a divine gambit, ending abruptly with a human-demon peace treaty. Both sides united to kill him for his heresy. Swearing vengeance against the gods, Leonard dies—only to awaken as Kael Veydrin, the talentless son of a Grand Duke in the Heizen Empire. His new life mirrors the old: a scheming sister, a weak body with clogged vessels, and endless mockery from nobles. Yet Leonard refuses to accept this cursed fate. Now inhabiting Kael's body, he vows revenge for them both. If the world branded him a heretic before, he'll show them what a true heretic can do—by dragging down those who rule from above.