WARNING: The following contains spoilers for Thor #19, on sale now from Marvel Comics.
During the climactic final battle of Thor: Ragnarok, a beaten-and-bloodied God of Thunder received a vision of his recently-deceased father Odin. Despondent over his perceived inability to defeat his sister Hela and the destruction of his enchanted hammer Mjolnir, Thor seeks his father's advice. Odin wisely responds by asking "Are you Thor, God of Hammers?" Although this line reminds the God of Thunder of his true power an unlocks the key to his eventual victory, it became a joke in the Marvel Cinematic Universe after the release of Avengers: Endgame, which saw Thor wield both Mjolnir and Stormbreaker against Thanos.
However, Thor #19 (by Donny Cates, Nic Klein, Matthew Wilson, and VC's Joe Sabino,) has just turned this beloved joke into a deadly new villain, introducing the true God of Hammers to the Marvel Universe. Although their true identity, motives, and abilities are currently unknown, the God of Hammers has already made a gruesome first impression by using Mjolnir to commit a horrifying atrocity against one of Asgard's closest allies.
In the aftermath of the cataclysmic War of the Realms crossover event, Thor finally succeeded his father as King of Asgard and the All-Father of the Ten Realms, achieving the destiny that has been promised to him from the moment he was born. Unfortunately, this new role has come at the cost of Thor's relationship with Mjolnir, which has become more and more strained throughout his ongoing series. Reasoning that his recent transition from a warrior prince to a king is to blame for Mjolnir's recent "behavior", the God of Thunder decides to leave the weapon at Avengers Mountain while he adjusts to his new role and the responsibilities that come with it. Soon afterward, Mjolnir vanishes without a trace, with even Sif, who has inherited her deceased brother Heimdall's role as the All-Seeing Guardian of the Bifrost, unable to find it.
Seeking answers, Thor reads a secret tome of Asgardian lore and discovers a prophecy centered around the mysterious God of Hammers, who is destined to kill the King of Asgard and plunge the Ten Realms into chaos. After rescuing his father from an intergalactic gladiatorial arena, Thor and Odin travel to the dwarven realm of Nidavellir, where Mjolnir was forged, to enlist their help in tracking down Mjolnir. There, they're greeted by the crushed corpses of dozens of dwarves and the shattered remains of the World Forge, the anvil that the dwarves used to create Mjolnir. Upon examining the bodies, an enraged Thor realizes that they were killed by the Mjolnir-wielding God of Hammers.
With this ruthless act of slaughter, the God of Hammers has delivered a deeply personal declaration of war to the newly-crowned King of Asgard. Over the years, Thor has made many powerful enemies, and any one of them could be the God of Hammers. Since Thor #16 (by Cates, Michelle Bandini, Elisabetta D'Amico, Wilson, and Sabino), the Thunder God has begun receiving sinister visions of his death at the hands of Thanos, who just so happens to be wielding an Infinity Stone-encrusted Mjolnir, making the Mad Titan a worthy candidate. However, it's also possible that this mysterious foe is an entirely new threat, someone who Thor has never encountered before.
If the God of Hammers is a new character, then their ability to wield Mjolnir is a terrifying preview of the power they must possess. Although Mjolnir is easily the most famous enchanted hammer in the Marvel Universe, there have been others, such as Beta Ray Bill's Stormbreaker and the numerous Hammers of the Worthy introduced in Fear Itself (by Matt Fraction, Stuart Immonen, Wade von Grawbadger, and Laura Martin). As the literal God of Hammers, this new enemy might be able to summon and control any of these hammers, breaking the enchantments that bind them to their wielders and claiming their incredible powers as his own. If that's the case, their growing influence over Mjolnir could explain why Thor's connection with his hammer was so volatile before its disappearance.
With Mjolner at their side, the God of Hammers is set to become a powerful threat to Asgard and its new King. Without his trusty weapon by his side, the Mighty Thor will have to find another way to face this new threat and avoid the ending to his legacy that has already been foretold.