Castamion is a region of Eldendil's first subterranean tier. Above this land of tunnels, caverns, and great fungal forests is the region of Draugluin. The following is a saying said to have its origins in Castamion:
When the surface dies, beneath it blooms.
It refers to the Draugluin Decay, the period when Castamion's plant-life goes into full bloom and expansion, while the land above goes into full decay. Over thousands of years, feeding on the nature energy from above, grew a vast verdant expanse clogging tunnels and filling caverns with abundance. At the center of all this is a dark jungle littered with ruined strongholds. Most were built in the Demon Spawn War (8972 GE - 9493 GE) when this region was contested by the night legions of Lolth and the demon generals of Goul'lumin.