In Le Morte d'Arthur, dreams take on the role of prophetic visions that describe future events. The prominence of prophetic dreams within the story demonstrates their importance to the culture of the Middle Ages. In those days, kings were considered representatives of God on earth; the concept of the divine right of kings meant that a monarch derived his authority from God alone. Therefore, it was considered fitting that he should dream prophetic dreams and be given heavenly visions of the future.
In Book 5, Chapter 4, Arthur dreams about a dragon engaging in battle with a boar. In the dream, the boar is the "foulest beast that ever man saw." The battle is an extremely violent one. The boar wounds the dragon and the sea is stained with the dragon's blood. The dragon emerges victorious, however, after it pulverizes the boar to powder. Arthur is disturbed by the dream and enlists a wise philosopher to interpret the dream for him. The philosopher explains that Arthur is the dragon and he will fight a tyrant (represented by the boar in his dream) who has been tormenting his people. The philosopher assures Arthur that he will emerge victorious, just like the dragon in his dream.
In Book 5, Chapter 5, the dream becomes a reality. Arthur battles a giant in the country of Constantine. The remorseless giant has been guilty of violent assaults on defenseless women; he has brutally murdered the Duchess of Brittany and thinks nothing of feasting on young children. Just as foretold in his dream, the battle between Arthur and the giant is gruesome and bloody. At one point, Arthur manages to cut off the giant's genitals and disembowel him. In the end, one of Arthur's knights cuts off the giant's head, and the giant is finally defeated. Arthur maintains that the giant is the fiercest one he has ever faced in battle.
In a vision in Book 21, Chapter 3, Arthur is visited by Sir Gawain, who warns him against fighting Mordred the next day. This vision is ominously followed by a strange dream Arthur has. In the dream, Arthur is sitting on a chair that is strapped to a wheel. Under him lies deep, dark water that is filled with "all manner of serpents, and worms, and wild beasts, foul and horrible." In his dream, Arthur topples into the water, and each of his limbs is seized by a beast. He screams in his sleep, and his knights rush in to wake him up. In the vision, Arthur is warned by Sir Gawain that he will die if he fights Mordred. Coupled with the nightmarish dream, the prospects can hardly be worse for Arthur.
In the end, Arthur does engage in battle with Mordred; he kills his arch enemy but is mortally wounded in the process. Although Arthur is rumored to have died from his wound, some people believe that he lives.
It can be seen that the dreams and visions are prophetic in nature; they divine the future for Arthur and are meant to be used as sources of wisdom. Interestingly, while Arthur ignores the last dream he has and pays the price for it, the author suggests it is still the prerogative of kings to ignore spiritual guidance that comes in the form of visions and dreams. After all, rumor has it that Arthur may have survived his wound. The implication is that, since a king rules by divine fiat, he will eventually be saved by divine grace, notwithstanding his mistakes.