Ilum was a terrestrial astronomical body smaller than a dwarf planet. With a kyber crystalline core and a frigid climate, it was considered a planet despite its size. The planet was situated in the eponymous star system located in a part of the Unknown Regions known as the 7G sector. Ilum was an arctic world; nearly the entire planet was covered in ice and snow, and was therefore inhospitable to most species, though it was populated with non-sentient animals.

Hidden beneath Ilum’s frigid surface was what came to be known as the Crystal Cave. Within the maze-like caves grew kyber crystals, which brought the Jedi Order to Ilum so they could harvest the crystals in a rite of passage known as the Gathering, in which Jedi initiates harvested their crystals by attuning themselves to the Force. As such, Ilum was a holy site for the Jedi Order for centuries.

Also located beneath the surface of Ilum were geysers, formed by volcanic activity on the planet. These geysers warmed some bodies of water under the ice and their size or duration could change due to volcanic episodes or mineral deposition. Geological formations were also present in the caves of the planet, formed over billions of years by accumulated mineral matter, deposited from hydrothermal fluids.

By 18 BBY, the Empire had already corrupted Ilum, draining its resources dry as well as having destroyed entire parts of the planet’s surface, as they were harvesting kyber crystals; the destruction was visible even from space. When Great Jedi Purge survivor Ahsoka Tano returned to the planet, she noted that there were at least two Star Destroyers and a mining ship in orbit around the planet. Chunks of the planet had been carved away, exposing the rock and magma that boiled up from the planet’s core.