The Prime Mover was the Ugor name for a prototype gravity well projector that the species scavenged from the wreckage of the first Death Star shortly after the Battle of Yavin. The piece of Death Star wreckage in which the projector was housed was known as the Holiest of Holies or Holy of Holies.
The Ugors toted the projector to their home, the junk-filled Paradise system, where their priest-scientists investigated it to find its use. When they turned it on, they found that it generated powerful gravitational energy. They tweaked it so that it could organize and order the refuse floating throughout the Paradise system. This enabled the Ugors to operate more efficiently and further surpass their rivals, the Squibs, in the galactic rubbish collection business. The species' religious leaders recorded the arrival of the Prime Mover in the Paradise system as a momentous occasion, which they dubbed the "Coming of the Prime Mover." The Ugors set up a picket line of Black Hole-class Salvage Dreadnoughts to protect the holy relic.
Sometime before the Battle of Hoth, however, a Rebel Alliance strike force and a Squib diplomat infiltrated the Paradise system and removed the gravity well projector. The system fell back into chaos as a result.
- The Complete Star Wars Encyclopedia