Hello. I posted this at FFN with the same title under the username MeliaWu, so don't be surprised if you found this story on that site.
Emma Watson as Hermione Granger/Princess Maya Nymeros Martell
Ton Heukels as Prince Rhaegar Targaryen
Gurbey Ileri as young Prince Oberyn Nymeros Martell
William Levy as Ser Edric Blackmont
Mehmet Gunsur as Young Prince Doran Nymeros Martell
Christian Serratos as Princess Elia Nymeros Martell
Ben Barnes as Ser Arthur Dayne
Bradley James as Young Jaime Lannister
Charles Dance as Tywin Lannister
Jason Isaacs as King Aerys II Targaryen/the Mad King
Samantha Smith as Queen Rhaella Targaryen
Maria Doyle Kennedy as Princess Arielle Nymeros Martell
Anthony Head as Prince Consort Doran Manwoody-Martell
Others:
Victoria Justice as Ashara Dayne
Robert Aramayo as Eddard 'Ned' Stark
Clive Standen as Brandon Stark
Aisling Franciosi as Lyanna Stark
Skandar Keynes as Benjen Stark
Henry Cavill as Robert Baratheon
Madison Davenport as Dorea Manwoody