Iokath was a massive sphere constructed around a star located in the Iokath system somewhere in the Unknown Regions, specifically Wild Space. This was the origin planet for the GEMINI droids, SCORPIO, the Eternal Fleet and the Gravestone.
Iokath's origins are shrouded in mystery. It was created by an ancient, highly advanced civilization that attempted to create the greatest weapons in the galaxy. To achieve this, they sent their droid armies to randomly selected planets, where they waged war on the natives, until the Iokathii had got what they wanted. They then offered help rebuilding the worlds they'd destroyed.
Some time after Iokath's construction, however, the Master Designer, the chief engineer, got tired of using their weapons in simulations, and longed for the real battlefield, testing it's power on Iokath itself. The armies destroyed the population, despite efforts to strand them in Wild Space and wipe their memories, until it was driven back by the Gravestone, a superweapon which was left on Zakuul after a battle against the Eternal Fleet. By then, the people on Iokath were destroyed and ARIES, a war AI designed to lead simulations, eventually took control over the planet and destroyed anything organic by putting them through impossible "tests".