The final battle, properly called the Battle of Camlann, pits Arthur and his knights against Mordred, his treacherous son, who has claimed the throne of England while Arthur was away, renewing grievances with Lancelot. Arthur returns upon learning of Mordred's usurpation of the throne, and sets to work driving him out.
However, the battles between the two sides are fraught with deep losses, and Arthur is pressed by a dream to make a brief truce. Mordred agrees to the truce, on the condition that both sides are free to break it if they see a sword drawn. The implication is that a drawn sword signals a renewal of hostilities.
Unfortunately, one of Arthur's men is bitten on the foot by a snake. The knight draws his sword to kill the snake, unwittingly breaking the terms of the truce and initiating the battle. The battle kills almost every single person on both sides, sparing only Sir Bedivere, who is left with a mortally wounded Arthur.
The fact that it's a snake which initiates the battle may be another one of many Christian symbols in the story. Just as the serpent (Satan) was responsible for the loss of mankind's innocence, a serpent was responsible for the battle and the subsequent end of Arthur's reign, heralding a return to darker times for England.