The Tarterian Depths of Carceri

The plane of Tarterus is located midway between chaotic evil and neutral evil in alignment. Each known layer of Tarterus is described as a chain of glowing scarlet orbs stretching into infinity. Each orb of the top layer is the size of a Prime Material world and the orbs get smaller with each successive layer. The orbs of the "inner" layers can be imagined as nesting within the orbs of the "outer" layers like Matryoshka dolls.  The Companions of the Hall visited this plane.

The six layers of Tarterus have a seemingly infinite number of worlds arranged like a string of pearls stretching into the air-filled void. In the top layer, Othrys, each orb is about the size of a Prime Material world and averages about 100 miles (160 kilometers) from the nearest orb. In each successive lower layer the planetoids grow smaller and farther apart. There is no sun but the soil of each orb gives off heat and a dull reddish glow, similar to the phosphorescence of fire beetles. The planets do not rotate and there is no day or night unless a greater deity wishes it so in their realm. Seasons and weather occur randomly and what little plant life there is  rapidly matures after rainfall and then withers hours later. Each orb is connected to one specific Prime Material world.

The barriers between layers were always located at the deepest points on each planetoid and always lead to the same sphere in the next layer, so the worlds of Tarterus can be imagined as spheres nested like Matryoshka dolls and each layer contained a string of orbs of a particular size