- "Remember. Remember, Anakin. Remember your home. Another galaxy. You were chosen. Remember?"
"YES! YES I REMEMBER! The beginning! 500 years ago, on the planet Ziost!"
- ―Anakin Skywalker, having a psychic conversation with Darth Ramirez
Ziost was the legendary lost homeworld of the Sith. As it turns out, everything we knew about the Sith was wrong, and they were an alien race that came from a silly planet in another galaxy, where everybody acted like extras from Dune. Only evil. Ziost was the capital of the Sith Empire... which one, nobody really knows. But anyway, like Korriban, Ziost was a planet resonating with evil and filled with all sorts of dungeons and evil artifacts and undead.
Apparently, Anakin Skywalker was not born on Tatooine, as everybody had previously assumed. Turns out he was from Ziost all along. Anakin was the Chosen One sent to the galaxy by General Katana in order to win The Prize. (Possibly this was the Jedi Heavyweight Title later won by Hulk Hogan of the New Jedi Order.) The only way Anakin could become the Chosen One was to chop off the heads of all the Jedi, which he attempted to do during Order 66. Due to massive dissatisfaction among the fans, this entire continuity was retconned by George Lucas, and later versions of the films eliminated all mention of the planet Ziost.
In the Expanded Universe, all the important Sith made pilgrimages to Ziost and discovered all sorts of interesting evil stuff. Lumiya built her lightwhip there. Galen "Starkiller" Marek visited Ziost and killed absolutely everything. Darth Bane smuggled holocrons from there. Jacen Solo also raided the place during his evil years. Even gasp! Good Ol' Whatshisname went there, where he...where he...nearly ate a girl!
The planet Ziost fell victim to Lord Disney, who made a new canon, erasing Ziost.