The Soul stealer was an ancient artifact that had the ability to take control of beings. There were many ways of manipulating the power of this artifact, by fusing this with a simple lightsaber or simply using the abilities of the Force. Once manipulated by the power of the soul stealer the manipulated beings would never return to their former self.
A milky-white talisman, the soul stealer belonged to the eccentric Jedi Humat over 19,000 years before the Battle of Yavin. Humat kept the soul stealer on Coruscant within his vault known as the Cell of Imu.