Something of a “catch-all” phrase for the many races that live deep in the earth, the underdark includes all kinds of subterranean dwellers. Drow (or dark) elves, duergar (gray dwarves), svirfneblin (deep gnomes), illithid (mind flayers), and even some goblinoid peoples. As a general rule, the further down you go in the caverns and chasms, the more insidious the cultures you will encounter. Even the most ardent explorer is hesitant to travel too deeply into the underdark, for fear of what might be lurking there.
The known access points to the underdark from Slothjemia are in the eastern portion of the Coreland, close to the lake city of Jaggerholmschloss. This has allowed a steady trade to develop between the underdark and Slothjemia, and by extension, the rest of the surrounding surface world. The primary contact is the drow, specifically the sprawling underground metropolis of Kaht-Medinhk, which is a weeks journey, entirely below the earth, from Jaggerholmschloss. There have been occasions when either the drow or the Slothjemians have asked for each other’s aide against invasion, and more often than not, those calls for assistance are honored. It cannot be claimed that the two have a formal alliance, but they do get along quite splendidly.
Further down, however, relations are not as cordial. There are the occasional members of underdark races that come to the surface, and adapt to life there (so-called “sundered” people), but that is as far as things go. Direct trade is rare with deeper races, but the drow have an excellent position as a middleman for trade between Slothjemia and everyone further down. This is very good news for anyone that has an affinity for spider-inspired artwork, trinkets, clothing, and pottery.