A Sith cave was located on the Outer Rim planet Mustafar. A place strong with the dark side of the Force, the cave resided in the course of one of the planet's lava rivers. Shortly after the rise of the Galactic Empire, the newly-christened Sith Lord Darth Vader traveled to the Sith cave to "bleed" a kyber crystal in order to construct his own lightsaber. Later, a castle was built over the ancient structure, which was designed to channel the cave's dark side energy and serve as the personal residence of Darth Vader.