Sometime associate and contemporary of The Architect, the Old One is the second entity to emerge from Nothing. The Old One displays considerable bitterness when speaking of the Architect, claiming that She refused to recognize his artistry in creation, despite their collaboration on at least three experiments. It is worth noting that of their three children, only The Mariner is referred to as the 'Adopted' son of the two; the Mariner has stated that the Old One sired the three of them on mortal women, who are later revealed to have been possessed by the Architect at the time of conception. The Old One's youngest son, The Piper, tried to free the Old One roughly seven hundred years before the events of the books, but was stopped by being thrown into the Void of Nothing. It is revealed in Superior Saturday that it was Saturday who had the Piper thrown into The Void, though the Piper thinks it was Sunday. If not for the chains with which he was bound, his physical strength, gigantic stature, and self-claimed mastery of the magics that can manipulate Nothing would allow him to make "light work" of his tormentors, clockwork puppets who remove his eyes, and do so again every time they regrow. The burst of Nothing in the Lower House had by Superior Saturday weakened the clock enough that the Old One destroyed his tormentors and will be freed eventually.
Similar to the Architect, the Old One can be described as a type of Nithling, as a Nithling is described as a self-willed being from Nothing.
When, in Lord Sunday, Arthur reunites all parts of the Will, the Old One is finally freed as Nothing cascades through the leaks in the Middle House, the bulwark of the Upper House, and two huge rips in the ceiling of the Incomparable Gardens, destroying his chains and setting him free. He, being extremely powerful, forges his way into the Dome of the Incomparable Gardens and joins up with the Will, effectively making the Architect whole and destroying the Universe as we know it.