Maggy prophesized to a young Cersei that
Cersei: When will I wed the prince?
Maggy: Never. You will wed the king.
Cersei: I will be queen, though?
Maggy: Aye. Queen you shall be... until there comes another, younger and more beautiful, to cast you down and take all that you hold dear.
Cersei: Will the king and I have children?
Maggy: Oh, aye. Six-and-ten for him, and three for you. Gold shall be their crowns and gold their shrouds, she said. And when your tears have drowned you, the valonqar shall wrap his hands about your pale white throat and choke the life from you.
Will the Valonqar kill Cersei? For this to be case, within the exposition of the show it should be clear that should Cersei be killed it be done at the hands of the Valonqar