Asmodeus: Birth of the Demon King

The Duke and the Duchess were blessed with a child after ten years of their marriage. The nurse informed the Duke about the safe delivery of the child. The Duke was delighted after hearing about the newborn but it didn't last long as he looked at the infant, agape in shock. The kid had a little horn bulging out, at the left side of his forehead. "What is this?" The Duke asked. "I-It's a h-horn, my Lord." The nurse answered. The Duke sat down on his knees and started checking the newborn's body. "This is not good." The Duke said as he looked at the right shoulder of the infant. It was an unusual birthmark. He had the letter A engraved on his shoulder, with some circular embodiments. "He is a demon child," The Duke said. "But that's not possible, dear. Both of us are humans and the demons have already been sealed away from our realm." The duchess said while lying on the bed. Is it possible for two humans to have a demon child? Or did the duchess had some illicit relationship with some demon in her last life? (just kidding) What will be the Duke's decision? Will he abandon the kid or does the infant has something else written in his fate?