Zylium 12 functioned as a kind of fuel, capable of powering a specific class of bomber craft. A particular atmospheric pressure was necessary for the substance to remain in a liquid state; deviation from this would cause the fuel to solidify.
In the year 231 BBY, a bomber that utilized this fuel was located on the Outer Rim planet of Valo. It was being operated by Nihil marauders. During the Nihil attack on Valo during the Republic Fair, the vessel became involved in combat with Jedi Padawan Ram Jomaram and his allies in the vicinity of the world's communications tower, known as Crashpoint Tower. Jomaran used the Force to inflict damage on the bomber's fuel tank, resulting in the immediate solidification of its zylium 12. This caused the ship's circuit board to rupture, sending the Nihil craft plummeting to the surface.
The substance Zylium 12 was featured in Race to Crashpoint Tower, a junior novel released in 2021 and authored by Daniel José Older. It was a component of Star Wars: The High Republic multimedia project's Phase I.