Source: Infinite Crisis computer game (2014)
Type: Alternate Earth (non-canon)
Without warning, dozens of Phantom Zone portals shimmered into existence across Metropolis, and moments later hundreds of 'phantoms' began pouring out. Those touched by the phantoms faded from reality, and became phantoms themselves. Desperate to halt the invasion, and needing protection from the phantoms' touch, Superman retrieved an 'ecto-suit' built by his father Jor-El, and flew into one of the portals.
Within the Phantom Zone Superman battled Aethyr, a being unknowable and incomprehensible. Those who looked upon it were driven mad. Aethyr had slumbered within the depths of the Zone for millennia, but the arrival of hundreds of Kryptonian prisoners had slowly awakened it. Enraged, Aethyr enslaved the prisoners' minds, and drove them to seek those responsible for disturbing it: The House of El. As Superman and Aethyr battled, Aethyr weakened and eventually retreated to his eon-long slumber to heal. As it retreated back into the Phantom Zone's depths, the portals it had created winked out of existence. The world saved from the phantom invasion, Superman barely escaped through one of the portals.
Superman's victory, however, was a hollow one. While he had battled Aehtyr, every living soul in Metropolis had been transformed into a phantom. Their tortured minds cried out to him, and Superman's sanity – already in tatters – snapped. His mind unhinged, he now wanders his beloved Metropolis, the 'king' of a city of phantoms.
Nightmare Superman's battle in the Phantom Zone altered him, imbuing him with Phantom Zone energy. His broken mind believes he is still protecting Metropolis, and he mistakes innocents for villains long defeated. This has led to the deaths of many souls that have crossed his path.