In 2265, during the height of the Psi Corps's war with the resistance the Psi Cops were able to crack some of former Director Vacit's time-locked files in a facility on Ganymede. Some seventy years prior, Vacit had mysteriously disappeared, leaving Earth space aboard a Centauri ship that dropped him off at the nearest jumpgate to Vorlon space, from which he embarked on a fifty eight light year journey in normal space at relativistic speeds. His files on Ganymede allowed the Corps to trace his course, believing that his destination was the place where telepaths had been created and held within it the secret to making more powerful teeps.
Diamond was given command of a Cyclops class cruiser to lead an expedition to find the secret and claim it for the Corps. Not long after their departure, a resistance team lead by Lyta Alexander managed to capture the Ganymede facility and learn of the Corps' objective. Lyta persuaded Michael Garibaldi to fund their own expedition in the hopes of beating the Corps there, or at least prevent them from getting what they were looking for.
Upon arriving at the outpost, Mr. Diamond and his crew found Vacit waiting for them and were quickly overpowered by the Nephilim under Vacit's influence. Forced to bend to his will, Diamond had his crew sent down, their ship abandoned and their shuttles destroyed. A short time later Garibaldi and Lyta Alexander arrived aboard the Toreador. Again using the Nephilim, Vacit fooled the newcomers, even Lyta that the Vorlons had succeeded in wiping out all life and then projected a telepathic image to make them believe that Diamond and arrived and killed Vacit before being shot himself along with all of his crew members.
After Lyta and Garibaldi leave, Diamond pleads with Vacit to let them go as well. Vacit tells them the he needs them here and they still have a lot of work to do