Sir Gawain and the Green Knight AD499

Sir Guaen, one of Arthur's most trusted Knights, is under attack from a supernatural being. The Green Knight is an unstoppable animated corpse hell-bent upon Guaen's death. The only two magical beings in Caerleon castle that have any hope of opposing this evil are powerless whenever the Green Knight is near.
Desperate for help Merlin turns to Queen Nimue of the faerie people. In an alliance forged from need the mystical beings unravel the circumstances that cause the appearance of the Green Knight at every full-moon.
With each ensuing attack the Green Knight becomes more powerful and more cunning. Can the alliance between the faerie people and King Arthur's court overcome their mistrust of each other in order to battle the very embodiment of evil?