Nidavellir is a system of caves, 2x2 map tiles in size, on the main Midgard map. It is connected to Jotenheim to the northeast, River Gioll to the southeast, and contains stairways that lead down to the Halls of Grynr (generated randomly in any cell) and the Armory (from the SE cell). Nidavellir can also be reached by dimension traveling to black.
The area is completely dark; your zone map will only display your local surroundings and will not remember which locations have been explored. You can map the area with cartography, but the mapped sections will also turn black as soon as they get within your normal line of sight.
Additionally, the area is quite dangerous. Many nasty monsters spawn in this region, including the draugr. Because of these two frustrations, and the fact that Nidavellir contains no important loot, visitors are recommended to spend as little time in Nidavellir as possible, using the region only long enough to visit the Armory. Note that the stairway to the armory is surrounded by stalagmites; either phasing or terraforming is required to access it.
If you make it past the stalagmites, you will find Uorik guarding the stairs. His damage per turn and AC are similar to that of a draugr, but he has only a fraction of the to-hit and total HP, making him far less dangerous to characters with low AC. He also has no special abilities, making him one of the least dangerous bosses, just a bit worse than Gymir.