WARNING: The following contains spoilers for Dark Knights of Steel #4 by Tom Taylor, Bengal, Arif Prianto, and Wes Abbott, now on sale from DC Comics
Dark Knights of Steel has been an interesting reexamination of the DC Universe, reinventing various figures from across the superhero universe. Sometimes, this means giving various characters new powers and identities, including one notable villain, who ends up being a fusion of multiple antagonists.
The version of Joker who was introduced in the world of Dark Knights of Steel is a brutal combination of the Joker and two other villains -- Lex Luthor and Sinestro. This quietly sets up this variant of the "cruel Joker" as one of the DC Multiverse's most dangerous Clown Princes of Crimes.
Dark Knights of Steel #4 revealed how the medieval fantasy versions of Jor-El and Lara became confidants of King and Queen Wayne. After recognizing the telltale signs of a nearby volcanic eruption, Jor-El warned the Waynes advisor Alexander Luthor. The prideful advisor refused to believe in a fate he didn't recognize, and his refusal to share Jor-El's warning almost led to the destruction of the town. As a result of his mistake, Luthor was banished from the kingdom and lost his title and riches. But after discovering a mysterious green power source later identified as a Green Lantern Ring, Luthor spent years mastering his newfound power before returning to attack the Waynes.
Driven mad by the ring, Luthor is revealed to effectively also have become Dark Knights of Steel's version of the Joker. He was described as the Green Man and the Cruel Joker. This version of Luthor is quietly among the most powerful variants of the Clown Prince of Crime in the multiverse. This isn't even the first version of Joker to have been fused with Lex Luthor or Sinestro. The Luthor who appeared in the Elseworld storyline Superman: Speeding Bullets ended up revealing himself as his world's Joker. Meanwhile, a Sinestro Corps empowered Joker recently appeared as a member of the multiversal Injustice Incarnate. But seeing a version of the Joker with Luthor's ruthless intellect and Sinestro's power ring means there's a Joker with the full potential of three different villains.
Even in a world that operates in the largely antiquated ideas of combat and technology, there are genuine powerhouses. A superpowered Joker with the wits of a Lex Luthor is the worst kind of wildcard who could exist at the moment, as the world of Dark Knights of Steel is already on the verge of chaos. The murder of Jor-El and the ensuing vengeful attacks by his daughter against Black Lightning and his son have set the stage for a massive superpowered conflict. And yet, this variant of the Joker still seems to be at play, with the villain likely the man behind Green Arrow's assassination of Joe-El. A Green Lantern Ring could be spotted in the sequence, suggesting a more familiar Lantern wielder like Hal Jordan or John Stewart. But Luthor finally enacting his revenge on Jor-El gives the sequence a much harsher element.
There have been powerful variants of the Joker before, with many appearing in various roles across the course of storylines like Dark Nights: Metal and Dark Nights: Death Metal. But the one in Dark Knights of Steel gains an edge by turning him into a Superman-level threat instead of just an enemy of Batman. The personal hate for the villain will be there for the Dark Knight of this world if he is indeed tied to the murder of Jor-El, but he's also capable of holding his own against Kryptonians and has the mind to devise terrifying plans. As war likely coming due to the violent events of the series thus far, the Green Man might be one of the best (and most powerful) versions of the Joker to appear in an alternate reality lately.