Where can I watch Merlin: The Return in United States?

When Merlin cast a positive spell to protect the knights of the Round Table, he used ancient magic drawing on the power of Stonehenge, and the knights were put into a sort of suspended animation. The evil Morgana and her son Mordred were banished into another world for 1500 years, but a 20th-century scientist finds a gateway, and the dark lord has a vicious scheme to enslave King Arthur's world.