Setting
Go delve down, however, and you enter "The Deep", the "true" under dark. Few descend into it's deep caverns, even the subterranean races that call the upper levels home. A deep dark expanse where few from the above world venture, the lower levels of the under dark is a cold, some times freezing, place completely cut off from sunlight. Strange creatures dwell within these deep caverns, becoming more alien and mysterious the further down you go. Rarely does an adventuring expedition trek down into The Deep, and the few that return always carry haunted expressions and tell tales of moving shadows and phantoms among the stone.
The dungeon lords themselves will sometimes hesitate to venture into the deep.
However, it is said that the mythical Nexus, the beating heart of the world itself, lies at the very bottom. It is speculated that all the magical ley lines are connected to the nexus in some way.