WARNING: The following contains spoilers for Avengers #34, by Jason Aaron, Javier Garron, Jason Keith, and Cory Petit, from Marvel Comics, on sale now!
In the previous issue of Avengers, a stoic Moon Knight attacked and stole the powers of several Marvel heroes: Iron Fist, Doctor Strange and Ghost Rider. He attempted to take Black Panther's power, but was rebuffed when he learned that those powers are in the Panther's blood.
Finally, Moon Knight confronted Thor on the moon. There, Moon Knight revealed an important part of Mjolnir's background: Thor's famous hammer is made of moon rock. The issue ends with Moon Knight using Thor's own hammer to bash the Asgardian around and knock him into space. With the Avengers' powers and weapons, Moon Knight's god Khonshu tells him they are now prepared to save the world.
Moon Knight and Khonshu have one of the most complicated relationships in the entire Marvel Universe. The god resurrected Moon Knight, aka Marc Spector, to serve as his avatar on Earth -- but over the years, he has taken great pleasure in tormenting Moon Knight. Khonshu enjoys being vague and appears to be much more of a schemer akin to Loki than a benevolent god.
In Avengers #34, the purpose behind gathering the power of Earth's Mightiest Heroes is revealed. Khonshu is calling upon Moon Knight, his avatar, to stop an incoming evil. Though the specific nature of the evil is kept vague, it begins with Mephisto.
Moon Knight approaches Mephisto -- who is in a magical prison cell in Las Vegas -- and accuses the Lord of Hell of orchestrating "dark events" from his cell like a supernatural mob kingpin. Moon Knight has a trick up his sleeve and sets Mephisto free, only to attack him. Moon Knight employs all the stolen Avengers' powers in a quick battle that ends with Mjolnir punching a rectangle-shaped hole through the demon's chest, seemingly killing him.
In order to properly "kill" Mephisto, Moon Knight will need all that power: Burn the body to ash with Ghost Rider's hellfire. Smash the ash with the Iron Fist. Use Doctor Strange's power to enchant whatever's left. Finally, use Mjolnir to scatter everything across millions of light years. It seems like a lot of work and Khonshu is pretty sure even all that won't spell the permanent end of Mephisto.
Still, Moon Knight knows what he must do. At this point Khonshu steps in and decides to dispose of Mephisto himself. To do so, he wields Mjolnir.
Until now, Thor's hammer hadn't been carried by Moon Knight or Khonshu. It just floated about, seemingly because of its moon rock composition and Khonshu's dominion over all things lunar. Seeing Khonshu actually hold the hammer is the shock of the issue, because it calls into question Mjolnir's "worthiness" clause, or, perhaps, hints at Khonshu's motivations or power.
Maybe Khonshu is being serious, that he really wants to save the world in an altruistic manner. Avengers #34 shows Khonshu's presence on the world is more oppressive than nurturing, but the control he's exerting could be part of a scheme that ultimately has humanity's best interests in mind. On the other hand, Khonshu's ability to use the hammer could be pointing to the god possessing much more power than previously shown. His ability to reforge the world in his image so quickly supports that interpretation.
There's also the matter of Mephisto, who is never actually shown having all his tiny bits spread to the four winds. Whatever direction The Age of Khonshu is heading, there is more than meets the eye here. The issue ends with Moon Knight revealing to Black Panther that he's aware Khonshu is not always as kind as he might be -- which potentially has big implications for the future.