The Architect is the creator of everything, including The House and Secondary Realms. The Architect first made the Incomparable Gardens and its Denizens, then created the raw matter of the Secondary Realms. She then made the Old One from her to look over Her creation from Herself. As told by the Old One, The Architect was the first thing to ever come out of the Nothing, though he did mention to Arthur that he was "hard upon Her heels".
During the events of the series, The Architect has been absent for the last ten thousand years, leaving behind the Will to carry out her wishes. For most of the series, her fate is unknown. During Drowned Wednesday, the Raised Rats told Arthur of a rumor going around that She might have been murdered by the Trustees but during Superior Saturday the Old One confirms that whatever happened to her was by her own will.
Only in the last book is the truth revealed. The Architect created the Old One out of Herself, as a part which could interact with Her creation more freely than herself. She has, in the course of an experiment, conceived three children with the Old One: Sunday, The Mariner, and The Piper. But as time wore on they began to grow separate and argue. She condemned the Old One to a cruel punishment for all eternity when She considered him to be "meddling" with the Secondary Realms, defying the Original Law.
When Her anger finally cooled, and she became tired of life, she realized that she could not destroy herself because She was not whole. In her anger She had made the Old One's bonds eternal, so the only way She could die was to commit her entire Creation to Nothing. However, the Will was broken by the Morrow Days into seven pieces, separated, and hidden because the Trustees had grown disobedient, and refused to commit themselves to the The Void. The Will warped them to her purposes, however, leading their various excesses and eventually the fulfillment of the Will. The Seven Parts of the Will, once combined, created part of the Architect, while the Old One made up the rest. Once recombined the Architect destroyed the last part of the House before finally allowing herself to die.