- "There is no place more sacred to the Jedi."
- ―Jedi Padawan Ahsoka Tano, regarding Ilum's Crystal Caves[src]
Ilum was an arctic planet located in the Unknown Regions. It was used by the Jedi Order for the Gathering, a rite of passage in which Jedi younglings must find and harvest kyber crystals for their lightsabers.
The planet Ilum was situated in the Ilum system, a star system located in a region of the Unknown Regions known as the 7G sector. Ilum was an arctic world; that is, the entire planet was covered in ice and snow, and was therefore inhospitable to most species. Hidden beneath Ilum's frigid surface is what came to be known as the Crystal Caves. Within the maze-like Crystal 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, Illum was a holy site for the Jedi Order for centuries.
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, this destruction visible even from space. When 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 lava that boiled up from the planet's core.