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The Keys to the Kingdom is a fantasy-adventure book series, written by Garth Nix, started in 2003 with Mister Monday. The series follows the story of Arthur Penhaligon and his charge as the Rightful Heir of the Architect to claim the Seven Keys to the Kingdom and the seven demesne of the House.

Arthur, a 12-year-old boy, has recently moved to a town and wants to fit into it. After suffering an asthma attack, he is saved by a mysterious metal object, called a Key, given by an even stranger character, , whose servants bring an incurable plague to Arthur's town. Arthur hurries to the House, a mysterious structure that only he can see. Shortly after arriving in the House, Arthur discovers the structure of the house is a complete universe and is informed of his duty to unseat the seven Trustees who run the House, claim their Keys, and rule all of Creation. Arthur cannot live an ordinary life unless he overthrows all of the Trustees, who are also known as the Morrow Days. To do this, however, he must use the Keys, which infect him with sorcery and make him a Denizen of the House; and whenever Denizens appear in the Secondary Realms (everything in Creation that is not in the House, including Earth), they are inimical to mortal life. This dilemma is a constant theme in the books: as Arthur does not wish to turn into a Denizen; he often resists using the Keys, and only does when it is absolutely necessary.

Arthur's current title as the Rightful Heir of the Architect is "Lord Arthur Penhaligon, Rightful Heir to the House, the Keys of the Kingdom and the Architect, Master of the Lower House, Lord of the Far Reaches, Duke of the Border Sea, Overlord of the Great Maze, Commander in Chief of the Glorious Army of the Architect, Master of the Middle House, Ruler of the Upper House, Lord of the Incomparable Gardens, the House and the Secondary Realms".

In Chapter One of Superior Saturday, it is implied that Arthur's use of the Fifth Key at the end of Lady Friday has completed his permanent transformation into a Denizen of the House: he grew taller and grew longer hair, looked better, and got stronger. Leaf and Arthur are returned to Friday's temporary hospital on Earth. Where Arthur starts to look at the crocodile ring on his right hand, which measures his metamorphosis from mortal to Denizen, he stops himself at the last moment, as "In his heart, he knew the answer without looking at the ring". This is confirmed when Arthur is now shown to have golden blood, and when he endures many physical attacks that a mortal would not survive as well as self-healing powers. His transformation also came with a negative side effect: his mind is sometimes plagued by thoughts normally associated with a superior Denizen (i.e. arrogance, contempt for those weaker than himself, anger, etc.), and he exerts great mental effort to suppress these thoughts. By the end of Superior Saturday, it is indicated that Arthur is at least 75% immortal, though it is hinted earlier that he is not necessarily a Denizen but may be something else.

Mister Monday has not been a contender in any CBUB matches.