Regulus

Regulus is the seat of the Primus, the ruler of Mechanus, set among an endless city of machines: gears, cogs and impossible machines in every direction, a twenty-sided fortress with a great gate, megafactories in a hemispherical ring, and light that seems to come from everywhere at once. Small winged, eyeball-like creatures go about their tasks in its spaces.

The fall of the Primus. A demon came to Regulus, killed the Primus and took his place in the energy pool where the Primus is seated. It was described as goat-headed and winged, working green magic, with the smell of death about it. Sixteen years before 1034 PC (about the time of The Skyfire, and not by coincidence) the demon left, one of its horns broken, and the way was open for one of the three secundi to be promoted to Primus. But long contact with the pool had let the demon's influence taint the new Primus, and perhaps a larger part of Mechanus. The other two secundi cannot ascend now that the first has; their bodies lie in the pool, radiating a little of the same green magic.

Orcus and Dhum. The goat-headed demon was Orcus. Dhum had been a secundus of Mechanus, a paragon of logic and law who secretly longed for freedom; Orcus offered him liberty in exchange for servitude, and Dhum fell and became his agent, building The Dreadnought of the Undying Sun and leading a corrupted modron march against Talonkeep. The modrons arming in the corners of Regulus are the first sign of that march.

Rebels and maruts. Rebels in the city told SAM about maruts, the inevitables whose purpose is to enforce the inevitability of death and, sometimes, the terms of certain contracts.

Modrons. The ancestors of all warforged probably came from here, carried across the planes over thousands of years by the great modron marches. The next march is not due for more than a thousand years, yet in the shadows and corners of the city modrons have been seen sharpening spears and making arrows.