Mustafar is a planet located in the Force Galaxy. It is a lava-based world, with an unstable atmosphere. It is basically one big volcanic eruption.
The Endar Spire's final resting place was located on this planet, making HK-1 the planet's first sentient inhabitant until, after centuries of failed attempts, the Techno Union was at last able to establish a mining colony on the volcanic planet.
Mustafar was the site of the lightsaber duel between Darth Vader and Obi-Wan Kenobi that left Vader hideously scarred. Because of this, the planet has a noticeable Dark Side aura. This energy was later harnessed by Tavion and her minions during her quest to reawaken Marka Ragnos.