WARNING: The following article contains spoilers for Justice League #12 by James Tynion IV, Frazer Irving, Bruno Redondo and Sunny Gho, in stores now.
Over the years, we've seen Batman utilize a multitude of different gadgets to fight in the name of justice. Housed in the Batcave are weapons designed to counter just about anything you can think of. Most recently, after being crippled by Lex Luthor, the Dark Knight has been utilizing a high-tech wheelchair a la Marvel's Professor Xavier, to help put his body back together.
In Justice League #12, with the Legion of Doom descending on the Hall of Justice looking for the cosmic power source known as the Totality, the Dark Knight kicks it up a notch, going after his enemies by unleashing yet another weapon. But this time, it's something that wasn't even designed by him; Batman's actually geared up with Lex Luthor's power suit.
The mecha suit is a bulky prototype that makes Batman look like he's wearing a slimmed-down version of Tony Stark's Hulkbuster armor. It's stocky and feels heavy duty, not entirely unlike the armor he wore in Frank Miller's The Dark Knight Returns, the Arkham video games, or Zack Snyder's Batman vs. Superman: Dawn of Justice.
While the League is trying to stop the floodwater infection in Drowned Earth from turning the world's citizens into sea creatures, Batman ends up using his enemy's suit to successfully defend the Hall from the Legion's nefarious looting squad: Joker, Grodd and Sinestro.
However, after Luthor wraps up his portion of the mission, he returns to steal his crew back from the Bat, but not before gloating how desperate the hero must be. Upon seeing Batman, he quips about how cute it is that the Caped Crusader kept one of his prototype suits. It doesn't seem likely Batman repurposed it, either. Instead, Batman appears to have donned it out of desperation, most likely requisitioning it out of desperation from one of the League's storage rooms.
It isn't revealed exactly when or where the League got this power suit. Lex has worn so many iterations, it's tough to pin down exactly which prototype one might be. However, it does seem to be a mix of the New 52 suit, the one from the Superman/Batman "Public Enemies" arc, and the oversized war unit he wore just before DC's Forever Evil storyline.
Luthor initially created the armor because he realized that in order to take down Superman and the League, he required brawn as well as brains. He armed it with teleporting tech, Kryptonite and anything he could think of to make him the equivalent of, or stronger than, Superman. This, of course, led to the blue armor he emblazoned with the Man of Steel's S-shield when he tried to turn good as a member of the League.
It's unlike Bruce to not have some sort of massive Bat-armor at the ready, but with everything going on in the DC Universe presently, we can forgive this lapse -- but only if it means the next time the pair clash, their properly prepared and suitably armored up for the confrontation.