The following contains spoilers for Earth-Prime #2, on sale now from DC Comics.
In the CW's Superman & Lois, fans have been eager to learn more of what happened on John Henry Irons' world. All they knew was that Superman broke bad, and along with a Kryptonian army, he razed Metropolis and other cities, bringing about the apocalypse. It's why John became Steel, with his mission becoming even more personal after Lois Lane (John's wife) was killed by Kal-El.
The show didn't elaborate on why Kal did what he did but come the second issue of Earth-Prime, the truth has been revealed. This prequel comic actually detailed the dark Superman's tragic origin, while confirming a scary enemy was on his way.
In this story, "Controlled Burn," (from Jai Jamison, Adam Mallinger, Andrew N. Wong, Tom Grummett, Norm Rapmund, Hi-Fi, and Tom Napolitano), Kal-El initially ended up with the Kents after his spaceship crashed. Sadly, they died in a car accident and Clark got tossed into an abusive home. Despite defending his new mom from a vile father, Clark was branded an outcast, so he fled. He'd end up being experimented on by the government which wanted to learn more about his powers.
Luckily, Kal would escape with his Kryptonian crystal and form a Fortress of Solitude in the Amazon. In this cave, though, Jor-El made sure to instill good in him, exorcising the hatred he had for how humanity mistreated him. It resulted in Superman wearing his black suit and becoming a savior, winning over the world with his altruism. No matter which government feared him, the public backed the Man of Steel.
Things went south, however, when his half-brother, Tal-Rho, arrived and used his forked tongue to corrupt Superman. Tal (Lara's son with Zeta-Rho) promised Kal a sense of family and belonging. All Kal had to do was join him and use the Eradicator machine to put Kryptonian souls into human vessels. Tal saw it as a small sacrifice because once they had this army, they could purge the planet of tyrants and make a paradise for both races. Superman, tired of humanity's love for violence and war, gave in to this warped vision, which led to the brief scenes the TV show had where he fried Lois and many others with his heat vision. The legion then pillaged Metropolis and Coast City, basically letting the world know who the new masters were. It culminated in Steel trying to kill Superman, only to get thrown through a portal and into the Arrowverse at the start of the show following "Crisis on Infinite Earths."
The issue ended with a shocking reveal, though, of Magog lurking in the shadows. In Kingdom Come and the mainstream DCU, he was found to be a lost soul, believing killing criminals was right. This ran him into conflict with Superman and many other heroes later on, but here, Magog's cutting a different figure.
The first issue had him using his staff, teleporting, recruiting, and powering up Clayface to form an army. In this world, he's more observant and prepping his next move with his godlike powers. Magog vowed to liberate society from the facade of superheroes, wanting to kill the likes of Batwoman. Thus, it'll be interesting to see if he recruits this evil Kal and Tal, or ends up feuding with them in what he views as a necessary rebuild.