if it's the one I think it is, it's the one from Ep III.
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 personal abode 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.