Warworld was created by the Warzoons, a warlike race who wished to rule the universe. It was bigger than a white dwarf star, the surface was covered with gigantic nuclear missiles and laser cannons capable of destroying entire planets, and all weapons were controlled mentally at the control chair. A peaceful race known as the Largas came to Warworld one day and found they had all died out mysteriously and the last one was sitting dead at the control chair.
The Largas buried the last one, and although they could not bring themselves to destroy Warworld, they used its crystal key to shut it down and as they all died out, the last one gave the crystal key to the Martians before dying out. Mongul sought out the crystal key to activate Warworld to take over the universe and coerced Superman into going and fighting the Martian Manhunter and obtaining the key to save his friends, whom Mongul had captured. Mongul gets the key and activates Warworld, and Superman and Supergirl seek him out to stop him.
They find that the weapons are too much for even them, but Superman, with his telescopic vision, realizes that the way the Largas were buried on Warworld would imply that they must have died out one at a time, and realizes that the one thing that could have killed them one by one was the control chair, and that the strain that it puts on the operator's mind is too much for them to handle, and so he and Supergirl force Mongul to draw out all of Warworld's weapons, overwhelming Mongul's brain and forcing him to flee. The two of them then break into Warworld and destroy it, ending it's threat forever.