In a multiverse full of heroes and villains, there was always bound to be villainous versions of Thor. One of them was Thorr Odinson from Earth-14325, where the Avengers were a force for evil. Thorr was actually evil enough that when he ended up on Earth-616, he killed some passing humans for not kneeling before him.
But how could an evil Thor wield Mjolnir if he would've been unworthy? The short answer is because he was unworthy that he was able to wield his Mjolnir, since the normal enchantment in the 616 was reversed. This hammer has a history with Thor during his phase of unworthiness, coming through in a clinch when our Thor needed it most. Being that Thor's entire life has been changed in the MCU, in many similar ways, could we see a similar multiversal mallet empower Thor when Jane Foster wields Mjolnir in Love and Thunder?
First debuting in Avengers #25 (by Jonathan Hickman, Salvador Larroca, Frank Martin, VC's Cory Petit), this evil version of Mjolnir is functionally similar to Thor's mainstream hammer. It has all of the same powers and components, but the phrasing on the hammer is slightly altered. One word in particular is changed in the inscription: "worthy" becomes "unworthy," meaning only those who are not pure of heart or whose bellies are filled with divine justice can wield it. This creates a difference for Thorr's Mjolnir from the original in that it can be wielded by a greater number of people who don't have to meet the rigid standard of worthiness usually required of Mjolnir.
Of course, there is the possibility that the hammer inherently makes people worse simply by virtue of what is required to wield it. The worthiness tied to Mjolnir ensures that only a select few can wield the hammer and be empowered by it. But those who strive to wield it usually go through a metamorphosis, becoming the most powerful version of themselves in their efforts to be worthy of such power. A hammer that requires only unworthiness means no one has to better themselves to wield such great power. It invalidates the lesson that people must strive to be the example to have so much power, and it could also corrupt people as gaining so much with no effort could make someone greedy or even vain, such as Thorr.
This hammer has, interestingly enough, been wielded by Thor of Earth-616. At the time, he was unworthy of the hammer thanks to a message delivered by Nick Fury on the moon that shook the thunder god's resolve. His celebrated journey to worthiness did eventually reunite him with his hammer, but the universe-threatening events of Marvel's Infinity event, curated by Jonathan Hickman, saw the depowered Thor Odinson in need of some godly strength. Luckily, the Avengers held onto Thorr's hammer after defeating hi previously, and our Thor was, being unworthy, able to heft this Mjolnir to empower himself with its dark magic. It was no substitute for his hammer, but it filled the void in the meantime. It was a great return for Thor after such a downturn in his fortunes, but will we see a similar turn in the MCU?
Phase Four has been broaching the topic of the multiverse after shows like Loki and films like Spider-Man: No Way Home. An evil Thor could easily exist out there. Should he leave behind his Mjolnir, our Thor -- who may no longer be worthy while Jane Foster holds the true Mjolnir for herself -- could use it in the real deal's stead. Granted, Thor has Stormbreaker now, but he could possibly lose or maybe even dual-wield both. If so, he'd be just as dangerous as he was when fighting Thanos at the end of Avengers: Endgame. It all depends if Marvel chooses to go down such a divergent path, but a little touch of darkness in Thor's life could go a long way in the MCU.