Furnace of Rath
The vast room is filled with hundred-foot heating elements that convert solid rock into a molten form for the production of flowstone. At the furnace's core, the temperature steadily rises until the lava is white hot, when it is sucked up into the stronghold far above. This is the font from which flowstone is formed, the molten birthplace of Rath.
Such enormous temperatures have to be vented, however, and even with the colossal ducts spewing heat and ash into the Cinder Marsh, stress fractures still form in the walls of the subterranean complex. These fractures often develop into large passages that can lead up to the massive network of caves surrounding the stronghold's main chamber. As their size and complexity increased over the decades, the passages became a security concern to the successive evincars of Rath. A by-product of the flowstone manufacturing process was funneled in to fill these cracks: a black, tar-like substance that has some of flowstone shapeshifting properties. The flowstone refinery featured dedicated channels that carried the black stuff to the caves when and wherever it was required. Most of the substance is ejected into the Death Pits. Like the flowstone, the black goo responds to the will of the evincars and automatically clears passages when necessary. It is also effective for the disposal of corpses, as it disintegrates flesh on contact.