On the eve of battle between Arthur and his knights and the forces of Sir Mordred, the ghost of Sir Gawaine came to Arthur in a dream and warned him to postpone the battle for one month if he wanted to win. So, the following day a feast was prepared, and both parties met and toasted the truce. But King Arthur warned his knights that he did not trust Sir Mordred, and so ordered that if any of Mordred's knights raised his sword, battle should commence immediately. Sir Mordred commanded the same of his forces.

Right so came an adder out of a little heath bush, and it stung a knight on the foot. But when the knight felt him stungen, he looked down and saw the adder, and then he drew his sword to slay the adder, and thought of none other harm. And when the host of both parties saw that sword drawn, then they blew trumpets and horns, and shouted grimly.
- Malory.

And so the battle was joined.

And never was there seen a more dolefull battle in no Christian land. For there was but rushing and riding, foining and striking, and many a grim word was there spoken either to other, and many a deadly stroke.
- Malory.