WARNING: The following article contains spoilers for Avengers #25 by Jason Aaron, Stefano Caselli, Jason Keith, Erick Arciniega, and VC's Cory Petit, on sale now.
Thor's hammer Mjolnir is the biggest, most well known hammer in the Marvel Universe, and possibly in all of comic books. In Avengers #25, however, the Asgardian God of Thunder is given a run for his money from a rather surprising source.
For the last three issues, the series has dealt with the Challenge of the Ghost Riders story arc, pitting Johnny Blaze, the Robbie Reyes Ghost Rider, and Cosmic Ghost Rider against each other. This forces a confrontation between the Avengers and Johnny Blaze, the King of Hell, leading to a battle of gavels.
Thor's Mjolnir vs. Ghost Rider's Hell Hammer
The Avengers attempt to aid Robbie Reyes, who has been challenged to a race by former Ghost Rider Johnny Blaze. Blaze's role as the King of Hell has apparently driven him mad with power, leading to his maniacal, antagonistic behavior. Before Blaze can finish barking orders to the denizens of Hell, Thor faces him directly in combat. He even pins Blaze into the ground by pushing his hammer on top of him.
Unfortunately for Thor, however, this only serves to give Blaze an idea. The restrained Blaze summons his bike, using his control of it to reformat the frame into a hammer. Being made up of the mass of Blaze's motorcycle, the mallet itself is far bigger than Mjolnir, as is its handlebar shaft. Sadly, the ensuing battle is mostly left off-panel, so the power of this Hell Hammer is never accurately measured against Mjolnir.
Ghost Rider & The Powers of Hell Hammer
Like Blaze's head itself, Hell Hammer is steaming with the fire flames of the underworld. This is a nice visual contrast with the billowing blue electricity of Thor's Mjolnir. Though Mjolnir is itself enchanted by Odin, the Hell Hammer, due to its actually being Blaze's Hell Cycle, is made of pure hellfire energy. Ghost Rider had always had control over his Hell Cycle, so Blaze being able to reformat it into a weapon isn't any sort of new, plot induced-element. Blaze is shown to be able to just as quickly transform the weapon back into its primary vehicle mode.
Due to being within Hell, it's possible that the Hell Hammer is even more powerful by virtue of being in its king's domain. The damage that it does to Thor, from what little of Thor and Blaze's fight is seen, seems to be more an annoyance than a true threat. The Hell Hammer briefly smacks Thor on the head as it is initially formed, though this rather minute looking damage may stem from either not being a direct hit, or from not being fully formed yet. Despite this lack of proof of power, there's certainly evidence that the Hell Hammer and its wielder could take Thor down.
Zarathos, the demon that gives Blaze his power, was once believed to possibly be powerful enough to defeat the Green Scar, one of the Hulk's most powerful forms. Surely Johnny Blaze could do the same to Thor, especially given his newfound role/power as the King of Hell. Hopefully, the Hell Hammer returns in future issues, as it and its wielder would likely be valuable assets in the hinted-at upcoming Ghost Rider war.