SPOILER ALERT: The following article contains massive spoilers for "Forever Evil" #6
If you've read "Forever Evil" #6, you will by no doubt have seen the surprise introduction of Alexander Luthor into the New 52. Perhaps unsurprisingly, the history of Alexander is quite a complicated bit of DC Comics continuity -- let's see if he we can untangle it a bit for you.
The first Alexander Luthor made his debut in 1982's "DC Comics Presents Annual" #1 in a story written by Marv Wolfman and drawn by Rich Buckler and Dave Hunt. In the old days of the DC Universe, there were two main Earths where stories were set. The most popular was Earth-1, where the modern superheroes lived (pretty much all DC comics were based on this Earth). The other was Earth-2, which was inhabited by the Golden Age versions of the DC superheroes (the original Superman, the Justice Society of America, etc.). A rarely-seen third Earth was inhabited by evil versions of the Justice League of America. This Earth-3 was home to the Crime Syndicate of America, and is where "Forever Evil" stars, the New 52 versions of the Crime Syndicate, originate from.
In "DC Comics Presents Annual" #1, the Lex Luthors of Earths 1 and 2 decide to switch places to take their best shots at the alternate versions of their main rival, Superman. In the end, both Supermen defeat their cross-dimensional rivals. They decide to imprison both Luthors in a limbo dimension. They break free, though, and head to Earth-3, where they decide to team up with that world's evil version of Superman, the known here as Ultraman. Both Supermen eventually make their way to Earth-3 where they meet that Earth's version of Lex Luthor, Alexander Luthor! And on a world with only super-villains, Alexander Luthor, of course, became Earth-3's first super-hero!
After helping the Supermen defeat the other Luthors, Alexander was not seen nor heard from again until Wolfman brought him back in the first issue of "Crisis on Infinite Earths" in 1985, drawn by George Perez and Dick Giordano. In "Crisis," the evil Anti-Monitor has been destroying entire universes, starting in the first issue with Earth-3, but not before Alexander Luthor and his wife, Lois Lane-Luthor, save their son, Alexander, Jr., by sending him away in an inter-dimensional teleporter, in a clever scene evoking Jor-El and Lara saving baby Kal-El from the destruction of Krypton.
Baby Alexander is rescued by the Monitor, who has been trying to defeat the Anti-Monitor. Soon it became clear that Alexander Luthor Jr. was a unique being; he quickly grew from an infant into a young man, and he apparently had the ability to control both matter and anti-matter. When the Monitor was killed, it was Alexander who took over as the head of the efforts of the remaining DC superheroes in their battle against the Anti-Monitor. At the end of "Crisis on Infinite Earths," when the Golden Age Superman manages to destroy the Anti-Monitor, the Multiverse was about to be destroyed and rebooted as a single universe, but before that happened, Alexander was able to reunite the Golden Age Superman with his wife, the Golden Age Lois Lane. Together with the Superboy from another Earth (Earth Prime, an Earth that is considered to be "our" Earth), the four heroes went to an unknown pocket universe as the DC Universe was reborn.
That was it for Alexander Luthor for well over a decade -- until Grant Morrison re-introduced the Crime Syndicate into the then-modern DC continuity in 2000's "JLA: Earth 2." Along with updated evil versions of the Justice League like Ultraman, Owlman and Power Woman, Morrison and artist Frank Quitely re-introduced a Post-Crisis version of Alexander Luthor, the only superhero of this other Earth. This time around, Alexander looks pretty much exactly like the "real" Lex Luthor, only he was a good guy.
Five years later, the Alexander Luthor from "Crisis on Infinite Earths" returned in DC's cross-company crossover, "Infinite Crisis," written by "Forever Evil's" Geoff Johns, with art by Phil Jimenez and Andy Lanning, and assists by George Perez and Jerry Ordway. As it turned out, Alexander went insane while in the pocket universe. He became obsessed with watching what was going on in the "new" DC Earth, stewing over how he could "fix it." He manipulated the Superboy who was with him to also hate this "new" Earth by showing Superboy only the bad things that had happened since "Crisis." Alexander also took over Lex Luthor's identity on a number of occasions to form a Society of super villains (and he also took over a spy satellite that Batman had created and helped to turn it into something much more aggressive and deadly), mocking the "real" Lex about how he is the same, only better.
Alexander's plan was to restore a number of previously destroyed alternate Earths and then merge them into one "perfect" Earth -- even if that means killing a whole bunch of people along the way. In his crazed state, he thinks that he is doing the right thing, that he is still "the only hero on a planet of villains." After the heroes defeat him, with the regular Superboy dying in the process, and Nightwing almost dying as well, Batman considers killing him. Wonder Woman convinces Batman not to, and then she, too, spares Alexander. He can't take this show of mercy; he believes that they are trying to say that they are better than him and he won't accept that. He escapes, but is tracked down by Lex Luthor, who is out for revenge, along with the Joker, the one villain Alexander never tried to recruit to join the Society. The Joker scars and then murders the one-time hero.
This Alexander makes one last appearance in James Robinson and Mark Bagley's "Justice League of America" #53 in 2011, where an alternate universe Green Lantern with the ability to temporarily resurrect people, brings Alexander back from the grave to give him a chance to partially redeem himself by helping the Justice League. He agrees, helping the Justice League stop the Omega Man.
And that was the last appearance of Alexander Luthor -- until today, of course. In "Forever Evil" #6, it was revealed that the Crime Syndicate's version of SHAZAM -- appropriately named MAZAHS -- was the Alexander Luthor of Earth-3, and the mysterious unrevealed prisoner in the Syndicate's custody. Which Alexander are we seeing now -- the original or Alexander Jr.? Only time will tell!