"Lord Sunday" is the final book in the Keys to the Kingdom series by author Garth Nix. It follows the protagonist, Arthur Penhaligon, as he confronts the ultimate challenges in the House and faces the powerful being known as Lord Sunday. The story combines elements of fantasy and adventure, exploring themes of responsibility and the nature of power. In this installment, Arthur must navigate complex relationships and make critical decisions that will impact both the House and the real world. The narrative culminates in a battle against Lord Sunday, highlighting Arthur's growth and the importance of friendship and sacrifice in overcoming adversity.