The Underworld is the realm of the dead, populated by fallen warriors who have never fully realized or accepted their passing. It is home to the Skeleton Army, also known as the Skulkin. The Underworld is a dark place, where land masses float in mid-air and are connected by bridges constructed by the Skeletons. Dragons and Skeleton vehicles are able to travel between the NINJAGO world and the Underworld. Once there was a passageway to the Underworld inside the Fire Temple, but it was destroyed by Kai. Lord Garmadon’s Dark Fortress is still located here, symbolizing the fall of the late Samukai who once ruled the Underworld.