WARNING: The following contains major spoilers for Thor #19, on sale now from Marvel Comics.
Thor has long been one of the mightiest heroes in the Marvel Universe, defying every obstacle that gets in his way with his unyielding strength and enchanted hammer Mjolnir. Unfortunately, the God of Thunder's connection to his mythical weapon has waned in recent months. While Thor himself has wondered whether this was due to his ascension to the throne of Asgard, the truth looks to be far more devastating than expected. Not only has someone else stolen his hammer, but they have just turned Mjolnir on its creators in vicious fashion. Whoever the culprit may be has certainly proven themselves worthy of Mjolnir just by being able to wield it, even though that worthiness might not mean the same thing it once did considering the circumstances.
When Mjolnir became too heavy for Thor to wield, the God of Thunder left it in Avengers Mountain in hopes that his connection with the enchanted weapon would improve over time. This provided an opportunity for an unknown thief to steal the hammer, leaving the king of Asgard furious. Worse still is the fact that not even the near omnipotent sight of Sif find where the hammer had been taken, forcing Thor to journey across the cosmos to recruit his father Odin to help in the search. The former king of Asgard led his son to Nidavellir in Thor #19 (by Donny Cates, Nic Klein, Matt Wilson, and VC's Joe Sabino) believing that the dwarves who created the weapon would be able to track it down.
Upon reaching the dwarven forge, Thor and Odin are overcome with grief at the sight of all the death and destruction laid bare before them. Even the massive Valraven who have stood guard over the entrance to the forge have been struck down, apparently without being given a chance to fight back against their assailant. The scene is a ghastly one, yet the broken bones and bent bodies are immediately recognizable to both Asgardians as the work of Thor's enchanted hammer. It's a startling sight that calls into question exactly how anyone could use Mjolnir in such an evil manner.
Regardless of Thor's might, it has never been his strength that allowed him to wield Mjolnir as his weapon. Only a person who is considered worthy has traditionally been able to even lift the hammer, but that doesn't seem to be the case at anymore. Aside from Thor's troubles with the hammer, the fact that someone was able to carry out such a gruesome attack with it might just indicate a whole new definition of "worthiness." Over the years there have been plenty of other astounding figures who have proven their worthiness by carrying Mjolnir in hand, yet none of them have ever turned it into a tool of wanton destruction as seen here. Assuming Mjolnir hasn't changed in recent months, the idea that someone truly worthy could carry out such an atrocious act could be a sign of the sheer power that the God of Thunder is up against.
It is hard to imagine anyone being worthy of the Mjolnir using it to massacre otherwise innocent beings, especially in their own home. If whoever currently wields Mjolnir isn't worthy, then the only other answer would be that the hammer itself has somehow been tainted.
Whatever the case may be, it is only a matter of time before Thor comes face-to-face with the culprit. The inevitable confrontation is sure to provide the God of Thunder with all of the answers he has been seeking, but he will first have to face an undeniably powerful foe without his enchanted weapon of choice.