Mustafar was a small planet located in the Outer Rim. The terrain was composed primarily of volcanoes and boiling lava rivers and fields, from which magma was harvested as a resource by the local miners. During the Clone Wars, the Techno Union established facilities on the surface to use the planet's rich but deadly resources to fuel the Separatist's war effort.
The planet also hosted a secret laboratory owned by Darth Sidious for experimentation on Force-sensitive younglings. However, when Anakin Skywalker and his Padawan Ahsoka Tano discovered the lab and rescued the children, Sidious had the facility's gravity supports deactivated to the building would fall into the lava river below.
The planet was also home to the base of operations for Black Sun. In 20 BBY, Darth Maul came to the base seeking the organization's allegiance to the Shadow Collective. After their refusal, Savage Opress decapitated the entire council, persuading Ziton Moj to agree to join their cause.