Radames pleads with the king to spare Aida and Amonasro . As his reward from the King , Radames pleads with him to spare the lives of the prisoners and to set them free . Gratefully , the King of Egypt declares Radames to be his successor and to be his daughter 's betrothed ( Aida , Amneris , Radames , The King , Amonasro , chorus : O Re : pei sacri Numi ! . .