The Keys to the Kingdom is a fantasy–adventure book series written by Garth Nix, published in seven books between 2003 and 2010. The series chronicles the adventures of Arthur Penhaligon, an asthmatic 12-year-old boy who is chosen to become the Rightful Heir of the House, the center of the universe. The core story line involves Arthur attempting to defeat the Morrow Days, the criminal Trustees of the House. The series takes place over a span of three weeks (time moves at different speeds at different locations in the series).
The Keys to the Kingdom are seven objects of power given to the Trustees along with command over their demesnes. It is unknown whether, but implied that, control of the Key is that which gives one control over a demesne and its namesake day in the Secondary Realms. The Key will bond to the rightful owner of a Key, and protect him or her from harm, somewhat from pain but not at all from discomfort. A rightful owner of a Key is an Heir, the Trustee to whom it was originally entrusted, or a Steward. Over time, the use of a Key will irreparably turn a mortal into a Denizen, though it is mentioned that after a few centuries, this process may reverse a little. Because the presence of a Denizen in the Secondary Realms is inimical to mortal life, Arthur attempted to use the Keys as little as possible, as he wishes to remain mortal and return to his family; however, he later realizes that without the keys he would not be alive, and used the powers to protect the Lower House and his home town.
The Keys hold sovereign power in their own demesne and their day. All other Keys are of equal power when they are not in their realm. The exception to this is Sunday’s Key, which is paramount over the others. They can do much of what is asked of them, such as opening, locking, manipulating, and freezing items. The full powers of the Keys are not known, as of Lady Friday. According to Arthur, the Keys have the ability to kill anything they wish, be it Denizen, mortal, or Nithling. Beings that are none of these, such as the Architect, her consort the Old One, their three sons, and possibly Arthur in later books, may be immune.
