Tsil, also referenced as either Spook-Crystals or Smokies, were mineral-based entities that had their habitat on the world of Nam Chorios. The names and nicknames for the Tsils were created by the Oldtimers, the descendants of the original Human colonists of the planet; these Oldtimers were not affected by the electrical storms that the Tsils generated. Those from off-world who were caught in the five to ten minute electrical storms produced by the Tsils would become ill for about thirty-six hours. The Guardian crystals were the largest variant of all the crystal types, giving them the ability to control the smaller crystals.
Luke Skywalker learned that Tsils possessed the ability to both reflect and amplify light and life energy. This amplification led to an increase in Force sensitivity on Nam Chorios, with even the smallest utilization of the Force resulting in an intense Force storm. The Tsils communicated through telepathy, placing images directly into the minds of others. Religious leaders of the Oldtimers, who were known as "Listeners," had the ability to perceive these "voices." It seemed necessary to be Force-sensitive in order to comprehend a Tsil. The Tsils issued a warning to Theras, the prophet of the Oldtimers, that the drochs residing on the planet were responsible for the devastating Death Seed plague. Because drochs are weakened by light, the Tsils' capacity to amplify light made them harmless on Nam Chorios.
Some Tsils were removed from the planet for purposes of commercial and industrial usage. Officially, they were designated to filter light for K-Class planets, but their true purpose was to act as the central element in Centrally Controlled Independent Replicant systems within the CCIR Needle and synthdroid projects conducted by the Loronar Corporation. The Tsils considered the reprogramming process used to utilize them for these purposes to be a terrible act. The Tsils offered Leia Organa Solo a solution to the Death Seed plague, but only if she ensured the return of all the enslaved Tsils. She agreed to this condition, and most of the Tsils were returned to Nam Chorios after the droch threat was contained.
During 44 ABY, Vestara Khai, a Sith apprentice, killed a Tsil in the midst of a battle against Abeloth, an entity of the dark side, in order to generate a death convulsion. This action caused harm to Abeloth, while simultaneously enabling Vestara and her Jedi allies to escape.