He was caught by Morgan when peeping on her during her bath. To prove his love, he wounded himself and passes on fake information to mislead King Arthur and his men.
He was under Sybil's orders to recruit people to attack the outpost on Bardon Pass to draw out King Arthur, and kill him.
He led a group of men that Wallace had recruited for him to attack Bardon Pass. After obtaining King Arthur's sword, he ordered a soldier to deliver the sword to Morgan while he stayed behind to torch King Arthur's body. However, Wallace had not managed to kill King Arthur. Harwel waited for reinforcement before attacking again. During the second attack, most of his men were killed by King Arthur and his warriors, who came back. When they let their guard down, Harwel spied an opportunity to shoot an arrow at King Arthur. However, Leontes jumped in and shielded Arthur. Harwel is killed by Gawain for causing Leontes's death.