After the death of General Grievous, the Separatist leaders were ordered to the Mustafar, a lava filled planet. There the Separatist Leaders meet their death at the hands of Darth Vader/Anakin Skywalker who Emperor Palpatine has ordered to kill them. Mustafar is modelled on Venus but on Mustafar there is a breathable atmosphere. The air on Mustafar is either produced by machines or there is some vegetation, Obi-Wan Kenobi and Anakin Skywalker fight to the death on the Mustafar. Obi-Wan beats Anakin and instead of killing his young padawan, he leaves Anakin to burn on Mustafar. Emperor Palpatine visits Mustafar to rescue his apprentice and take him back for medical care and turn him into Darth Vader.
Whilst most of Mustafar was filmed in CGI because of the dangers of Lava, there were some scenes and inspiration from Mount Etna, Scilly, Italy. The actors would be added in after the scenes were filmed.
Unlike Venus, Mustafar has a native race of life forms, the Mustafarians who live in caves and mine the lava. The planet is not an easy planet to live on with all the toxic fumes produced by Lava. In billions of years time when the Sun begins to expands as it nears its life, the Earth will turn into Mustafar. At first the temperatures will rise and the water will boil away, the vegatation will degrade and the atmosphere will be inhospitable. Unless we have found a way off this planet by then, the human race will end. Only the simplest of life-forms, micro-organisms will be the sole living creatures on this planet.
In addition to being introduced to it in Star Wars III - Revenge of the Sith, we also get to see it in Star Wars - Rogue One. Darth Vader has a castle built on a lava stream. You'd think anyone would want to get as far away from an incident as possible but Darth Vader is different.
Director Krennic comes to visit Darth Vader as he fears for his life from Palpatine. Darth Vader doesn't take too kindly being visited by Krennic and nearly strangles him.