Sir Giles of Clewsby was an evil knight and enemy of King Arthur. Through the magic of the sorcerer Black Barton he could transform into the Barghest, a "fearful night-beast." Those who were bitten by the Barghest were cursed to that they would gradually weaken and die within six hours. Black Barton followed him around and forced his victims to come to Clewsby Castle and swear fealty to him in return for having the curse lifted. At Black Barton's behest, Sir Giles took on the Silver Knight in single combat and, taking advantage of his refusal to wear his enchanted armor in a tournament, transformed into the Barghest. He kidnapped the now-cursed Silver Knight and imprisoned him in his castle. The Silver Knight would die unless Arthur surrendered his crown.