Mustafar was a small volcanic planet located in the Mustafar system, situated between two gas giants in the Outer Rim Territories that was aligned with the Confederacy of Independent Systems during the Clone Wars and later taken over by the Galactic Empire after the Techno Union was nationalized at the conclusion of the conflict.
During the Imperial Era, Mustafar became a fortified Imperial world as it housed the personal castle of Darth Vader.
Mustafar was a scorching lava world located in the Outer Rim Territories, and had an array of valuable minerals which were mined by the Techno Union, and was the headquarters of the infamous Black Sun during the Clone Wars.
The planet's native Mustafarian species lived in underground caves created by lava fleas as they ate through the planet's crust, and made their armor from the heat-resistant shells of the creatures. Only venturing up to the surface in order to collect valuable minerals from the lava flows, the short and strong southern Mustafarian subspecies carried out the heavy labor involved, while the tall and slender northern Mustafarian subspecies acted as guards and expert lava flea riders. Due to the planet-wide lava surface, all buildings were held up by gravity supports. Its capital city was Fralideja.
In the era of the Clone Wars, the planet bore witness in a plot to kidnap Force-sensitive children during the Clone Wars by bounty hunter Cad Bane, who deposited the children at a facility there for a Sith plan to create an army of Force spies. However, the children were rescued by Anakin Skywalker and Ahsoka Tano, and the facility was destroyed, along with any evidence of who was behind the plot.
The Black Sun fortress
During this period, the Black Sun criminal organization, under Xomit Grunseit, was headquartered on the world. In 20 BBY, Darth Maul, his brother Savage Opress, and Pre Vizsla, the leader of the terrorist organization Death Watch, traveled to the Black Sun fortress with the intention of recruiting them into their criminal empire, the Shadow Collective. Ziton Moj greeted them upon their arrival, and led them to the Black Sun's leaders. After negotiations with the leaders went sour, Savage beheaded all of them with his lightsaber. Moj became the new leader of the Black Sun, and joined with Death Watch, providing them with weapons, troops, and supplies.
At the war's end, Darth Sidious ordered General Grievous to send the Separatist leaders to a mining facility on Mustafar for their protection, but in reality to gather them together so that Skywalker, newly anointed as Darth Vader, Sidious' new apprentice, could kill them all, and deactivate the droid armies, bringing an end to the war.
Age of the Empire
Shortly after all Separatists were killed, Mustafar became the location of a duel between Vader and Obi-Wan Kenobi. Vader lost his remaining limbs and suffered severe burns all over his body. He was later rescued by Sidious and rebuilt at the Grand Medical Facility on Coruscant.
After the end of the Clone Wars, the Galactic Empire took control of Mustafar and maintained secret facilities on the planet where Jedi survivors of Order 66 were interrogated and executed. Rumor had it that Darth Vader carried out these interrogations, torturing the Jedi for any useful information before their execution. Additionally, both Vader and Sidious often returned to the planet, using it as a focus for meditation on the power of the dark side of the Force.
Four years before the Battle of Yavin, an Imperial fleet was stationed above Mustafar commanded by Grand Moff Wilhuff Tarkin aboard his Imperial I-class Star Destroyer the Sovereign. During a mission to rescue Kanan Jarrus, the Sovereign was destroyed during a duel between Jarrus and the Grand Inquisitor when his lightsaber was cut in two by Jarrus and fell into the ship's power core, sending the vessel falling towards the lava fields below.
In 0 BBY, Darth Vader had a castle on the very planet where he had been disfigured. Shortly after an insurgency on the desert moon of Jedha, Vader was in a bacta tank, out of his armor, when his servant Vaneé admitted Director Orson Krennic, who wanted to discuss the workings of the top secret Death Star.
After the Battle of Endor, Wedge Antilles went on a scouting mission to suspected Imperial planets. He reported back to the New Republic when he was on Raydonia before going missing. Due to its close proximity to that planet, Admiral Gial Ackbar sent a scout to Mustafar, as well as to Tatooine, Dermos, and Geonosis, while two scouts were sent to the planet Akiva.