The Us TV retelling of the Arthurian legend - a 10-episode Irish-Canadian production which will air early next year - also stars Eva Green as Morgana le Fey, Jamie Campbell Bower as King Arthur, Tamsin Egerton as Queen Guinevere, James Purefoy as King Lot and Clive Standen as Gawain.
Fiennes said: "The magic lies in the political essences of the piece.
"There will be dark arts. You'll see things disappearing and changing shape, but it's really about the birth of a legend, about power given to the wrong individuals, a country split by war and greed... and the ascent of a pure king and noble minds.
"It's a beautiful, riveting, romantic, sensational epic story that has never been told."
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