A soul snare was a particular type of prismatic crystal that possessed the special ability to trap a person's mind and Force essence for eternity.
The shamans belonging to the Order of the Terrible Glare, a breakaway group from the Jedi Knights, were responsible for the development of soul snares. These crystal prisms were used by the Order to imprison the Force essences of their adversaries.
Borborygmus Gog, a Shi'ido, merged technology he acquired from a Swarm-class battle droid of the Ssi-ruuvi with soul snares, resulting in the creation of the Essence Stealer.
Luke Skywalker came across a collection of soul snares just prior to his mission to the planet Garn. It was on Garn that he encountered Rur, who was the High Shaman of the Terrible Glare. It was said that the crystals could contain a sentient being's life force forever. Even though Skywalker had previously heard about these objects, he was unaware of their existence until that moment.
Rur utilized his computer-body to detect the proximity of individuals wielding a lightsaber. His intention was to capture and imprison Jedi as revenge for the Jedi Order's defeat of his splinter group.