Kalakar Six was a moon with Dark Side nexus which was once occupied by the Sith Empire. It was located in the same system as Dromund Kaas, the location of the Dark Force temple of the Prophets of the Dark Side.
Rumors held that around 100 BBY, the moon's dark side nature attracted Baron Yorn and his followers from Coruscant, who set up a fortress with the intent to conduct experiments there. However, a supply ship later noted that everyone there had vanished, except for Yorn, who had gone mad, tearing out his own eyes and tongue.
Just prior to the Battle of Yavin, Darth Vader came to the moon in an attempt to intercept the Rebels carrying the Death Star plans. The Rebels were not there: Vader had been lured to the moon with false information by the local darksiders, who pitted Vader against a doppelgänger of Darth Maul. Vader killed the ersatz Maul. Emperor Palpatine soon arrived on the moon in a shuttle, killing the darksiders and retrieving Vader. Vader suspected the entire ordeal had been a test arranged by the Emperor, but his master denied this.