Here's why this moment constitutes the climax:
* Rising Tension: The entire book builds up to this moment. Gwydion, along with the young King Taran, has been on a perilous journey to retrieve the Stone of Annwn, an artifact of immense power. Their journey has been fraught with danger and filled with difficult choices, and the stakes have been steadily rising.
* Confrontation: Gwydion's encounter with the King of the Shadowlands is a direct confrontation of good against evil. The King, driven by his desire for power and revenge, seeks to exploit the Stone's magic to destroy his enemies. This battle represents the culmination of the conflict that has been brewing throughout the story.
* Turning Point: Gwydion's victory over the King of the Shadowlands is a turning point in the narrative. It marks the moment when the forces of evil are vanquished and the Stone of Annwn is secured. It also signifies Gwydion's transformation from a young warrior to a mature and powerful leader.
* Consequences: The climax has lasting consequences. It sets the stage for a new era of peace and prosperity in the land of Prydain. It also reveals the true nature of the Stone of Annwn, demonstrating its potential for both good and evil.
Therefore, the climax of *The Stone* is a multifaceted event that brings together the threads of the story, resolves the central conflict, and sets the stage for the future.