Tyber Zann had been arrested on Felucia after stealing a Sith artifact from Jabba the Hutt. He sent Urai Fen and the artifact ahead of him, intending to follow him and meet up later. He did not get the chance. Imperial forces caught up with him and arrested him, shipping him off to Kessel.
Urai Fen escaped and returned to Ryloth, where the Zann Consortium had a base. Urai Fen gathered the Zann Consortium's forces, and they defeated Imperial forces on Kamino. After the battle, Fen laid low until Zann was ready to escape. Eventually he hired Han Solo and Chewbacca to transport him to and from the prison in the Millennium Falcon.
Meanwhile, Zann remained in prison. A mouse droid belonging to Jabba arrived and projected the image of the Hutt to Tyber. He taunted Tyber and demanded to know where the artifact was. Tyber refused to tell him. After he kicked the droid out, Tyber contacted Urai, who was nearby. Soon after, a bounty hunter sent by Jabba tried to kill him. Urai saved his life and they began their escape.
Tyber had to make it through locked doors by blasting the controls. As he made his way through the prison facility he fought stormtroopers guarding it. He also convinced many prisoners to join him.
Eventually Tyber ran into opposition by two Imperial dark troopers. He took this opportunity to rush to the Falcon while the prisoners engaged the droids.
He arrived at the Falcon, where Han and Chewbacca had been hired to take the two off planet. Han complained that this wasn't exactly what he had been told—Fen had told him it was a spice run—but Tyber offered him more money. They left, leaving the prisoners behind.
Very likely the prisoners were recaptured or killed, as no other transports were available for escape. Tyber Zann returned to Ryloth and began his revenge against Jabba and other plans.
Inspired by Zann's riot, the prisoners in the spice mines of Kessel staged another revolt after the Battle of Endor.
The presence of dark troopers at this conflict before the development of the Dark Trooper project even began could possibly be explained as these being early prototypes, or simply some other type of Imperial combat droid since they were not called dark troopers by name.