The prison guards were a force active during the Last Great Time War. They controlled a prison which held beings from across time and space, such as the Malmooth Chantir and a Sea Devil. They also guarded the Great Key.
The Eighth Doctor infiltrated the prison to take back the Key. After a month of imprisonment, he faked his death and then overpowered the guards when they checked on him. He took the key, freed the other prisoners and escaped. (COMIC: The Forgotten)