Following the misadventure of the adder the final battle was begun,

And thus they fought all the long day, and never stinted, till the noble knights were laid to the cold ground, and ever they fought still, till it was near night, and by that time was there an hundred thousand laid dead upon the down.
- Malory.

King Arthur stood up among the dead bodies and saw Sir Mordred alive in the distance. He asked Sir Lucan for his spear, and Sir Lucan warned him to leave Mordred alone because of the premonition. But King Arthur would not listen, and was determined to seek his revenge.

Then the king gat his spear in both hands, and ran toward Sir Mordred, crying, Traitor, now is thy death day come. And when Sir Mordred heard Sir Arthur, he ran until him with his sword drawn in his hand. And then King Arthur smote Sir Mordred under the shield, with the foin of his spear throughout the body more than a fathom. And when Sir Mordred felt that he had his death's wound, he thrust himself, with the might that he had, up to the bur of King Arthur's spear. And right so he smote his father Arthur with his sword holden in both his hands, on the side of the head, and the sword pierced the helmet and the brain-pan, and therewithal Sir Mordred fell stark dead to the ground. And the noble Arthur fell in a swoon to the earth.
- Malory.