Sigan-Kûn is a vast Underdark grotto sited in central Faeglor. Long ago, it was much smaller in size, and served as a floodplain for the frequently flooding waters of the Favánë. When the Nermanis Sea drained into the Underdark, it's onrushing waters greatly expanded this long northerly cavern.
A century after the Nermanis flood, when this area was no longer underwater, it was claimed by the Mulun-Dûna. The soil, now greatly enriched, was farmed Myconid, mushrooms and other foodstuffs. After experiments with dangerous types of pixie dust, they ended up tainting the waters of the Favánë. This in turn created a subterranean forest, a fungal called Erochel.