A soul snare was a prismatic crystal which had the power to encapsulate the mind and Force essence of an individual for all of eternity.
Soul snares were developed by the shamans of the Order of the Terrible Glare, a splinter sect of Jedi Knights who utilized the crystal prisms to imprison the Force essences of their enemies.
The Shi'ido Borborygmus Gog combined technology that he obtained from a Ssi-ruuvi Swarm-class battle droid with soul snares to create the Essence Stealer.
A collection of soul snares was found by Luke Skywalker just before his mission to the planet Garn, where he encountered Rur, the High Shaman of the Terrible Glare. The crystals were said to be capable of holding a sentient being's life force forever, and although Skywalker had earlier heard of these objects, he had never been aware of their existence until that point.
Rur had used his computer-body to detect if lightsaber-wielders were near his location. In this way he desired to ensnare Jedi and imprison them, in vengeance for the Jedi Order's defeat of his splinter faction.
These prismatic gem-like objects were first mentioned in the comic Blind Fury!.