Final answer:
In 'Le Morte d'Arthur', Merlin asks for King Uther's firstborn, Arthur, in exchange for his help, which sets up Arthur's destiny as the future king.
Step-by-step explanation:
In Le Morte d'Arthur, Merlin asks for King Uther's firstborn, Arthur, in return for his assistance in helping Uther to be with the woman he loves, Igraine. Merlin's demand is based on ensuring the fulfillment of a prophecy concerning Arthur's future.
King Uther agrees to Merlin's terms, leading to the future king's birth and Merlin's role as his mentor. This is a pivotal moment in the Arthurian legend, setting in motion the events that would lead to Arthur's rise as the legendary ruler of Camelot.