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 Sigan-Kûn. After this calamity, Sigan-Kûn came under ownership of the Nazgarun. They used the now fertile soils of the area to farm myconids and plant mushrooms and other foodstuffs. When this empire experimented with dangerous types of pixie dust, they ended up tainting the waters of Favánë, creating periodic feydark fissures. This led to a quickening of fungal, mushroom, flora and fauna growth.
A very large and bio-diverse forest called Erochel covers much of Sigan-Kûn. This forest is home to all manner of normal and unusual Underdark plants and lifeforms. The unusuals ones are from Kriav, either descendants of them, or recent arrivals.