Complex sentence: In the present, the world is in mourning over Superman, whose death triggers the Mother Boxes to reactivate and Steppenwolf's return to Earth in an effort to regain favor with his master, Darkseid.
Split into simpler sentences: In the present, the world is in mourning after the death of Superman. This triggers the Mother Boxes to reactivate and Steppenwolf's return to Earth in an effort to regain favor with his master, Darkseid.