Apokolips is the home planet of Darkseid, and the seat of his power from where he plans his conquest of free will.
Apokolips is an ecumenopolis with burning firepits, extant in between dimensions, occupying a 'frequency' somewhere between the physical universe and Hell; the war that destroyed the Old Gods and created New Genesis and Apokolips separated the Fourth World from the rest of the universe proper, leaving it accessible by Boom Tube.
Apokolips is briefly seen in Chapter Twenty-Two of the Injustice Comic, where Darkseid converses with his son Kalibak over Superman's sudden declaration of peace on Earth, all the while observing the Black Racer being tortured. Apokolips is later revisted when Lobo goes there to challenge Darkseid to a battle.
Role in Injustice
Though not a stage, Darkseid's throne on Apokolips is briefly seen during a stage transition from the Hall of Justice.