Conker meets Gregg.
The Underworld is the basic limbo of the Conker universe that can only be seen once in the Game Mode of Conker's Bad Fur Day and Conker Live & Reloaded where you meet Gregg on your first death. In the Nintendo 64 version, it is a simple dark room with nothing interesting in particular, while the Xbox version has skulls, lanterns, and tombstones.