The Horizon Base heist represents an operation undertaken by the Spectres with the aim of acquiring fuel for Phoenix Cell. This fuel was to be taken from Horizon Base, which was under the control of the Galactic Empire, all in the year 3 BBY. Despite the mission achieving its objective, the rebels' astromech droid, Chopper, was inadvertently abandoned at the depot during the escape. He was originally guarding the starship Ghost while his companions secured the fuel. However, he spotted a droid leg for sale at a nearby stall by an Ugnaught vendor that matched one of his own mismatched legs. While haggling for the part, the others departed. Subsequently, Chopper absconded with the leg and fled Horizon Base aboard an Imperial freighter, where he both befriended and liberated the Imperial protocol droid AP-5.
Following the loss of their primary vessel, the Phoenix Home, due to an Imperial assault, the rebel faction known as Phoenix Cell initiated a search for a suitable location to establish a permanent base. During this period, the Spectres, who are members of Phoenix Cell, had stolen a Quasar Fire-class cruiser-carrier from the Empire while on Ryloth. This vessel was then repurposed as their new command ship, which the rebels christened the Phoenix Nest.
Ultimately, by synthesizing all available intelligence, including maps obtained from Lasat refugees Chava and Gron, alongside data provided by bounty hunter Ketsu Onyo, an old acquaintance of Spectre Sabine Wren, Phoenix Cell determined that the habitable moon orbiting the planet Berzite within the Yost system would serve as an adequate location. However, the Phoenix Nest was running critically low on fuel. Consequently, the Spectres were deployed aboard their starship Ghost with orders to procure additional fuel, enabling the fleet to undertake the journey.