Apokolips is the planet ruled by Darkseid.
Darkseid plotted his invasion on Earth with Desaad on Apokolips. During the battle Superman was captured by Parademons and taken to Desaad through a Boom Tube. While in Desaad's lab he explained that he wished to experiment on Superman and to integrate his Kryptonian abilities into a new breed of Parademons. Batman eventually arrived via Boom Tube to free Superman, however he found Superman had been driven mad by Desaad's machines and saw him snap Desaad's neck. He then moved to attack Batman. Superman & Batman later arrived back on Earth via Boom Tube and joined the rest of the league in defeating Darkseid and his forces, sending them back to Apokolips.
At some point, Knockout escaped Apokolips and fled to Earth.