The duel between Arthur and Valiant occurred during the final round of the Tournament of Camelot.
Events
When Arthur and Valiant advanced to the final round of the Tournament of Camelot, Merlin told Arthur that Valiant was using a magical shield to cheat. Arthur believed him and brought the matter before the court, but they were unable to prove their claim after their witness died, and Arthur was forced to go through with the duel to avoid allegations of cowardice.
The duel was fierce. At one point Arthur knocked Valiant's helmet off and removed his own to keep the fight fair, much to the delight of the crowd. Soon afterward, Valiant knocked Arthur to the ground, stepped on his shield, and disarmed him. He then pinned Arthur to a wall and Arthur shoved him away.
At this point, Merlin cast an animation spell to bring the snakes on Valiant's shield to life, exposing his secret to the crowd. Valiant responded by ordering the snakes to kill Arthur, but Morgana threw Arthur a sword and he killed both the snakes and Valiant before either could harm him (Valiant).