Kiss Me, My Mafia Brother

Adrian de Valois is a son of the de Valois family and was once known in the imperial capital as a prodigy. His stepmother Beatrice and his half-brother Hector frame him for a crime, and he is sent to the Demon Crystal Prison. He spends three years there among the most dangerous prisoners in the kingdom. In that time the primordial god Chaos blesses him, and he subdues demon after demon inside the walls until even the Sword Saint, the Dragon Mother and the Voodoo King are wary of him. Near the end of his sentence his fiancée Vivienne comes to the prison to break off their engagement, calling him a failure. He signs the papers without argument. Then a letter arrives from Colleen, the girl he grew up with. Her father Joseph has been jailed on a false treason charge, and Hector is using Joseph's life to force Colleen to marry him. Adrian breaks the collar around his neck, punches through the prison wall and returns to the capital. He gets Colleen away from Hector and enters the kingdom's Divine Blood Tournament, meaning to clear his dead mother's name. When envoys of the giants break into the arena he drives them out with chaos magic, and Chaos appears before the whole kingdom to name Adrian his heir. Adrian is crowned champion and raised to Grand Duke. He cuts himself off from the family that betrayed him, exposes what Beatrice did, proves that Hector committed the crime he was jailed for, and gets Joseph released. He and Colleen are formally engaged. Hector is sentenced to life in prison. Beatrice will not accept the loss. She joins with the giants and they attack the capital, and Hector breaks out of prison changed into a monster called a Son of Typhon.