Warworld is a technologically advanced artificial planet, which is essentially used as a giant weapon to conquer or destroy other worlds. The planet is entirely covered with technology and weapons. It has very few inhabitants. Warworld has been owned by both Mongul and Mongul 2
A warlike species, the Warzoons, created Warworld.
Cyborg Superman nearly succeeded in turning Earth into a new Warworld, with the help of the new Mongul. As a first step, he destroyed Coast City and replaced it with advanced, technological edifices unique to Warworld. This new city was known as Engine City. The city was destroyed by Superman and his allies.
The second Mongul created his own Warworld, which was destroyed by Imperiex, who also threw Mongul to Earth.
Brainiac 13 converted Pluto into a new Warworld prior to the Imperiex War as shown in the Superman: Our Worlds at War storyline. He used this Warworld to devour Imperiex. However, a powered-up Superman was able to destroy this Warworld, sending it to the moment of the universe's creation. (Superman: Our Worlds at War)