The duel between Arthur and Helios occurred during Arthur's Second Rebellion of Camelot.
Events
After Morgana discovered that her magic was being blocked and fled from the room, Helios took charge and faced Arthur in her stead.
Though Arthur managed to parry many of Helios' blows, his fighting was hindered by his injured ribs and Helios quickly gained the upper hand. He disarmed Arthur and knocked him to the floor by punching him in the face, then raised his sword to kill him.
Before he could deal the final blow, however, Isolde came to Arthur's aid and stabbed Helios in the back. Helios ultimately died of this wound, but not before he dealt Isolde a mortal blow by sweeping around and slicing her across the abdomen (The Sword in the Stone).