A hollow technological world containing its own internal sun, as well as its own Broken Metal and Fire Patterns used to power it. It was discovered by Lian during her travels in Shadow, and it became her base of operations when not in Amber. High in both technology and magic, it served admirably as a location for Lian to perform the majority of her experiments and construction efforts. It was also discovered during Kanna and Lian's adventures on Jade that Mechanus was one of the sole remaining constructed artifacts of the Nomad Artifacers, who had used the hollow world as a massive shipyard to build their trans-shadow Arks.
Sadly, during the HErald Crisis, the majority of Mechanus's mass was forcibly vaporized by the combined efforts of Lian, Kanna, Motoko, Kuonji, et al in trying to stop the Black Unicorn Amber Construct War Train. However, the Sunheart (the Broken Fire Pattern) and the Metal Rune (the Broken Metal Pattern) of Mechanus are still intact, as is the majority of Lian's Alpha Complex (which was built around and above the Metal Rune) and her construct [Vector Prime]? which is now in orbit around Mechanus's sun.
With Lian's untimely demise in the Abyss, control of Mechanus defaulted to Vector. One can only guess at what he's been up to for the past century, as he doesn't talk with the other Constructs much anymore.