Ziost, sometimes referred to as the "Gateway to the Empire" in the days of the Old Republic, was a world made up of ancient dark forests and barren tundra. However, the trees withered away and the planet became very similar, in landscape, to Korriban. Much of the planet was covered in ice.
Not only was it a focal point of the dark side, but it was also the adopted homeworld and Imperial capital of the Sith species, who left their original homeworld, Korriban, as a graveyard world after the death of their leader Adas.
Eventually it became a major Sith buffer world during the days of Vitiate's Sith Empire before being forgotten entirely.