The Prism, also called the Ghost Prison in Imperial reports, was a secret Jedi-run prison facility whose existence was only known to the Jedi Council and had been constructed during the Second Great Schism. The prison was used to house incredibly dangerous criminals during the Clone Wars and was located in the mass shadow of the sixth moon of Diab in the Diab system on the fringe of the galaxy, a three day trip from Coruscant, even with a presumably fast hyperdrive.
Diab's electrical storms wreaked havoc with sensors and no transmissions were allowed in or out of the prison. The prison was built to be inescapable and self-sustaining, and had a skeleton crew of 100 security droids and one Jedi Master, Master Skein. The prison also boasted a top medical facility.
The prison was visited by Jedi Master Obi-Wan Kenobi during the war, and then after the war by Darth Vader, Laurita Tohm, and Trachta while finding a place to regroup and treat the grievously-injured Emperor Palpatine. The prison housed 208 prisoners by then, half of whom had been apprehended by Anakin Skywalker and one who had disfigured Trachta. Darth Vader, alongside Trachta and valedictorian Laurita Tohm, were forced to locate the facility after a military coup severely injured Emperor Palpatine. Tohm approached Vader with the idea that releasing the prisoners would provide them with an army, but Vader amended the idea, believing that the prisoners should battle one another for the right to join the Empire. Of the 207 prisoners released, only thirty-three of them survived in fighting condition.