| Category | Caverns |
| Continent | Brucrumus |
| Region | Imgangreth |
| Map | Sidari-Nyx Passage |
Sidari-Nyx bears the names of the two Svirfneblin sibling-explorers who first mapped this region. In 1206, they published their detailed findings and cartographic records of this expansive Underdark passageway. These maps subsequently reached the surface world. Recognizing the commercial viability of the area, the Orchish Empire utilized the information in selecting the construction site for Valangrim.
In the Underdark, the Sidari-Nyx passage commences westward of Phazgamir. From the north, the passage's initial major obstacle is the Hlond Stor lava flow. This area hosts a fire giant town and a series of grand arched bridges. Bigmar's giants also operate lava barges across the river. Though thrice the cost of the bridge route, the barges offer a swifter and more effortless crossing. Approximately eighty miles south of the Hlond Stor lies the Umgaril river, a product of upward water movement from the depths rather than downward drainage. 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 grazes the shores of the Cthorgo sea. Until the Third Epoch, the passage terminated roughly forty miles south of this coastal region. In 1503, Matak's commerce department hired an Underdark mining company to link the Great Tradeway with the Sidari-Nyx, a task accomplished in under a year using a device called the Earthspear.