The 13-episode season is to kick off on Syfy Jan. 4 after wrapping up its run in the United Kingdom.
"As Season 5 opens, Camelot is at the height of its golden age," a synopsis said. "With Queen Guinevere and the Knights of the Round Table at his side, Arthur has never felt stronger. But even as Camelot flowers, the seeds of its destruction are being sown. As the forces of darkness gather, only Merlin stands between Arthur and a fate that has been written since the dawn of time."
Joining the cast this season is Alexander Vlahos as Mordred. Season 5 guest stars include Lindsay Duncan, Janet Montgomery, Liam Cunningham, Josette Simon, Sophie Rundle and Anthony Head.