Merlyn knew he had not warned King Arthur of something that was of critical importance, some piece of information that would greatly reduce the conflicts and difficulties Arthur would have to face while Merlyn was gone. However, it was difficult and confusing to try to keep track of the events that had already happened and the events that were going to happen in the future; Merlyn was becoming increasingly distracted by the approach of his encounter with Nimue and he "can't remember whether it is in the future or in the past."
When Merlyn finally realized what he has forgotten to tell Arthur -
It was Arthur's mother's name which he had forgotten to mention in the confusion!...Arthur's mother was Igraine-that very Igraine who had been captured at Tintagil.
it was too late. Merlyn was on holiday far from Arthur's court, it was the middle of the night, he was exhausted after a busy day of watching the wildlife, and he was "muddled with his backsight, and dreams were in his noddle. He...could not remember whether he was in the future or the past"
Merlyn went back to bed, and Arthur awoke to find Queen Morgause, his mother's daughter, his half-sister, standing in front of him "folding up a tape... and looking her best." Unwarned and inexperienced, Arthur could not resist the charms and magic worked by his half-sister.