Dead in Seven Days. Reborn in Time to Win?

The prince of the kingdom went missing as a child. He was found years later and brought back, and the king named him heir and fixed the wedding and the coronation for seven days later. During the years he was gone the king had taken in an adopted son with the same face as his. Leona, who carries the blood of the Elf King, was matched with the prince. The adopted son killed the prince and buried the body in the forest. He put on the prince's identity and presented himself for the ceremony in his place. The prince came back inside an egg, with a system called Dream Fear attached to him. It runs on fear he causes, and enough of it will put him back in a human body before the seven days are up.