- "Although the Clone Wars were over, some people never seem to get the message. The worst case of denial was Gizor Delso, a Geonosian Separatist who'd somehow escaped Lord Vader's efforts to wipe out the remnants of the CIS. Lord Vader tracked him to Mustafar, and brought us in to finish the job."
- ―Unidentified 501st stormtrooper
The Holdout on Mustafar was an rebellion that took place in 18 BBY. It was an short-lived rebellion that was led by one of the last Geonosians, Gizor Delso, and was backed by several small-time Separatist Holdouts in the Outer Rim.
The holdout was also using an old Separatist Droid Foundry/Mining Facility on the lava planet Mustafar to create a Droid Army much like the original Separatist Droid Army from the Clone Wars with B1 Battle Droids, B2 Super Battle Droids, and more. The Empire quickly discovered this and ended up launching an attack on the Holdout using stormtroopers from the 501st Legion to destroy the droid forces on the planet and eliminate Gizor. Although the Separatists gained the upper hand for a time as they had a massive fleet of warships the Empire didn't know about, the 501st Legion killed Delso and evacuated the base/factory/mining facility then orbitally bombarded it from space using an Imperial Star Destroyer.