The Hero Becomes Duke's Eldest Son

Story:

Aden Albireo was called “The Dragon Slayer,” the hero who slayed the Berserker, and the Demon Lord, who ultimately, saved the world. But he was betrayed by those who swore friendship and was left for dead… “I never thought I’d come back to life!” 15 years before the Demon Lord’s resurrection, The reckless eldest son of the Duke, who was unheard of and unseen, “Aden Albireo” has returned in the body of “Aden Remes”! And what’s more… “Long time no see, Dragon King!” The Demon Lord I thought I had slain is within my soul?! Will Aden be able to end his uncomfortable cohabitation(?) with the Demon Lord and prevent his resurrection?