Answer:
The author wants to show the crisis that is upon Lancelot and the emotional turmoil he is going through within himself. This is achieved through the author's detailed description and perfect imaging of the finest details even though it was nighttime.
Step-by-step explanation:
Sir Lancelot returns to Camelot after another successful adventure. Even though he is favored by the King and also is famous, he is not happy. Being famous and successful isn't the only thing that can keep man happy, it seems. He and Queen Guinevere had been having an affair behind the King's back and the King doesn't even have an inkling of it. He still treats Lancelot with the same treatment he had always did, with love and everything that is deemed fit for a knight. But with the queen stuck in a loveless marriage and their illegal relationship, Lancelot feels a heavy heart and can't even be happy with the queen. He knows he's betraying the king's trust but he can't do nothing about it too. He's in a conflict between his loyalty for the king and his love for the queen.
The author, in showing this particular scene where Lancelot is seen bidding goodnight to the king wants to highlight and emphasize the emotional turmoil Lancelot is in and his inability to come to any possible solution without hurting either of the two other parties involved. And through the detailed description and the perfect display of the details (even though it was dark), he is able to show the emotional strain and turmoil Lancelot is in. And we as a reader, are deeply involved in that turmoil and can share in his agony