Crystalfen Caverns

The Crystalfen Caverns are an ancient, massive natural cave network located on the western shore of Tal'Dorei near Emon, extending below the Ozmit Sea.

Description
The Crystalfen Caverns are extensive, deep, and highly dangerous. Located along the Bladeshimmer Shoreline, they were settled by creatures from the Far Realm, aberrations such as the aboleths who built the now-fallen empire with Salar as their capital. There are entrances to the cave network at various points above the surface, and adventurers have mapped some of the caverns.

The psychic energies from the Far Realm gave rise to the mineral azuremite, a blue crystal that, like residuum, can be used to create suude. Because the aberrations and psychic phenomena, as well as the more mundane cave-ins, make the Crystalfen Caverns incredibly treacherous territory, as of 812 PD most of the people who entered the caverns did so only to mine azuremite for suude production.

One known entrance to the caves is from the Undervault catacombs, below Emon's Cemetery District.

Background
An ancient society of aboleths and other aberrations sprung up from a rift between Far Realm and Material Plane. They conquered the subterranean caves, drove their slaves to construct Salar, and slowly pressed upward toward the surface. However, their intentions crumbled when the final battle of the Calamity sent powerful waves of magical force throughout Exandria, causing much of the underground caverns to collapse and the aboleth civilization to fall under rock and rubble. Fragments of the city still remain slowly rebuilding from the dust.

Campaign One: Vox Machina
On their first visit to Emon, several members of Vox Machina fell into the Crystalfen Caverns. Grog, Tiberius, Vex, Percy, and Keyleth slipped into a subterranean river system that dumped them onto an island in the middle of Crystalfen Lake, where they were attacked by a resident aboleth. They escaped and ended up in ancient ruins where they found the hag Trysta. In exchange for clearing out a nearby umber hulk, Trysta offered to tell them where to find an exit from the caverns. The adventurers fought the umber hulk, who nearly killed Grog by tossing him from the ceiling of the cavern to the island below.