Sidari-Nyx is named after the two Svirfneblin sibling-explorers that first mapped it. In 1206, they published their findings and maps of this wide Underdark passage. These maps eventually found their way to the surface. Seeing the commercial aspects of the area, the Orchish Empire used them in selecting the building site for Valangrim.
In the Underdark, the Sidari-Nyx passage begins west of Phazgamir. From the north, the passage's first major obstacle is the Hlond Stor lava flow. In this area is a fire giant town and a series of great arched bridges. Bigmar's giants also run lava barges across the river. Though at three times the cost of the bridge route, the barges provide a quicker and easier crossing. Roughtly eighty miles south of the Hlond Stor is the Umgaril river. This river is more a product of water being moved up from the deeps, than one of water draining down from the land above. Another eighty miles south, travelers enter a fungal forest as dense as any surface jungle. One hundred and fifty miles south of this forest the Sidari-Nyx brushes the shores of the Cthorgo sea. Until the Third Epoch, the passage came to a dead-end roughly forty miles south of this coastal area. In 1503, Matak's commerce department hired a Underdark mining company to link the Great Tradeway with the Sidari-Nyx. This was done in under a year using a device called the Earthspear.