It is the homeworld of Legion foe Tyr, who was the ruler of this planet.
Tyr had the planet cannibalized and transformed into a mobile world, which then travelled throughout the galaxy attacking other planets.
One of The Controllers is tasked with stopping it, and begins construction of a Sun-Eater to destroy it. These plans are foiled when a Legion squad, not knowing its intent, destroy the Sun-Eater factory. The Controller then captures these Legionnaires, explains the situation, and forces them to help him in destroying Tyrraz.