Vexxtal
Very little information is available regarding the history or nature of Vexxtal Crystals. These crystals are typically located exclusively within the lightsabers wielded by practitioners of the dark side who draw upon the Force. Those struck by lightsabers containing this crystal are afflicted with a tainted presence.
Behind the scenes
According to an interview with W. Haden Blackman, this crystal represents the remnants of the "Force Plague," a capability that was ultimately removed from the completed version of The Force Unleashed.