How do circumstances frustrate Arthur's attempt to heed the warning from his dream?
In Book VIII of Le Morte d'Arthur, Arthur is warned in dreams about his death. When he does try to heed the warnings of the dream, circumstances interfere in the form of a serpent Arthur encounters as he meets to form a truce with Mordred's army on the battleground. Disturbed by the snake's appearance near him, Arthur tries to kill it with his sword, and his motion is misinterpreted as a signal for battle.
Book VIII describes Mordred as having made himself King of England, claiming Guinevere as his queen; however, she has escaped to London's Tower. After enlisting many Englishmen on his side, Mordred meets Arthur at Dover in battle. There Sir Gawain is mortally wounded. As he lies dying, Sir Gawain admits that his pride has caused him his fatal injury. Before he dies he writes to Sir Lancelot, beseeching him to come to the aid of Arthur, requesting also that the knight come to his funeral.
On the night before the battle with Mordred at Baredon, Arthur dreams that he is on the Wheel of Fortune while he sits upon a throne of the richest gold. Under him in this dream is a "hideous deep black water" that is at some distance. This water contains all types of "serpents, worms, and horrible wild beasts." Suddenly, the wheel turns upside down and Arthur is among all these horrible creatures. Arthur wakes crying out "Help! Help!"
Having learned of this prophetic dream, Sir Gawain and ladies from the court urge Arthur to postpone his fight with Mordred. They plead with him to at least wait a month before engaging in battle because by then Sir Lancelot will arrive. Heeding this warning, Arthur calls for a meeting with his enemy; the two armies meet on the field in order to set the terms of a truce. Unfortunately, as they stand on this field, a snake appears before Arthur. Without thinking, Arthur draws his sword to kill the adder, but the other side misinterprets his action as a signal to fight.
After a long battle, in the darkening evening, Mordred is the only man standing on his side, and Arthur is left standing with only Sir Lucan and Sir Belvedere. Then, Mordred and Arthur fight; Mordred is killed by Arthur, but Arthur receives a mortal wound in the conflict, just as has been predicted. Mournfully, Sir Lucan and Sir Belvedere bear him to the chapel.
In Malory's Morte d'Arthur, Arthur is warned by Sir Gawain in a dream that if he fights Mordred the next day, he will die. Arthur plans to seek peace the next day instead of fight, in order to give Lancelot time to arrive with reinforcements.
Unfortunately, while the armies are facing each other and discussing peace, an adder (snake) "stung a knight in the foot." When the knight draws his sword to slay the snake, both sides see the sword and blow their "beams (a kind of trumpet), trumpets, horns" and shout "grimly." The battle is on.
Arthur manages to kill Mordred, but he is also killed in the process, as was predicted in his dream.