Katak crystal

Little was known about Katak Crystals or their origins. Are only found in the lightsabers of dark side Force users and Sith. When prepared properly, this crystal can be used to drain some of the life force of a victim within close vicinity, enhancing the power of a lightsaber it is placed into. Initial exposure to one of these crystals can cause fatigue and ennui; prolonged exposure to the crystal cause death with the life force bolstering the power of the crystal.

Citations

http://starwars.wikia.com/wiki/Katak

Star Wars Force Unleashed

Star Wars Force Unleashed II