Final answer:
Bedivere casting Excalibur into the lake signifies the end of King Arthur's rule and the fulfillment of prophecy, with a mystical hand catching and brandishing the sword before taking it underwater, marking the return of the sword to its supernatural keeper.
Step-by-step explanation:
When Bedivere finally casts Excalibur into the lake, it marks the end of King Arthur's sovereignty and the fulfillment of a prophecy. A mystical hand emerges from the water, catching the sword and brandishing it three times before pulling it beneath the surface. This act signifies the return of the legendary sword to its supernatural custodian, reflecting the timeless cycle of birth, death, and rebirth in Arthurian legend. The enchantment associated with Excalibur comes to an end and with it, the era of Arthur's rule.