The following contains spoilers for Avengers #64, now on sale from Marvel Comics
The Avengers have handled all sorts of threats over the years, but the threats at the heart of their war against Mephisto have taken things to a whole new level. On top of dealing with the Council of Red, the Avengers have been forced to fight nightmarishly powerful versions of their greatest enemies in a multiversal version of the Masters of Evil -- one of whom just got a horrifying upgrade.
This version of Thanos is a member of the Multiversal Masters of Evil and briefly gained control of the Venom symbiote in Avengers #64 (by Jason Aaron, Javier Garron, David Curiel, and VC's Cory Petit). This turned him into a big enough threat that could have easily overwhelmed the Avengers if it hadn't been for the last-second intervention of a Marvel Cinematic Universe icon using his greatest tool in a pretty perfect way.
Thanos Bonded With the Venom Symbiote
While a multiversal army of Avengers has been doing its best to hold back a horde of Mephisto variants from claiming the God Quarry, a more personal battle has been taking place in the ancient past of Earth-616. The Multiversal Masters of Evil's attack against Earth-616 has united the ancient Avengers with their modern-day equivalents. The battle initially seemed to be going against the heroic forces of Earth-616, but the sacrifice of the ancient Ghost Rider was able to help turn the tide. After days of non-stop combat, the Avengers are able to finally wrestle an advantage away from the villains -- with Tony Stark defeating a villainous variant of his father and the alliance finally bringing down the Destroyer Armor-enhanced Killmonger.
Although the heroes are able to exorcise the symbiote off of the Black Skull with the help of a Phoenix-infused blow from Captain America, things quickly risk getting out of hand all over again when the alien flees -- and finds Kid Thanos. The symbiote quickly bonds to him, creating a horrifying hybrid. This amplified Thanos is enough of a problem for the mighty sorcerer Agamotto to be left nearly helpless -- with only the intervention of Thor, Odin, Namor, Captain Marvel, and Nighthawk keeping him from instantly killing the sorcerer. Venom proves to be incredibly difficult to separate from Thanos, but the combined might of Namor and Captain Marvel is able to wrestle enough of it off of him for the heroes to deploy a perfect solution. Both wielding Mjolnir, Odin and Thor simply drop their hammers onto a piece of the symbiote that had been swirling off of Thanos' body.
Thor's Oldest Trick Saved the Marvel Universe
A key element of Mjolnir in the Marvel Universe is the enchantment placed on it years ago. Only those worthy enough to lift the hammer can even move it, let alone use it. With both Thor and Odin doing a job that only one was likely needed for, the pair easily pin the symbiote and weaken Thanos long enough for Phoenix to throw her burning body at the villain. The resulting explosion splits them apart and defeats Venom, stopping Thanos from carrying out a planet-wide slaughter. It's a clever concept and one that plays on the history of Thor.
There have been plenty of times in the past when Thor has used the enchantment placed on his hammer to defeat his foes, effectively pinning them to the ground under the weight of the unmovable hammer. This makes Thor the perfect Avenger to have on hand. Thanos getting his hands on Venom is a worrying prospect, as the alien lifeforms' habit of enhancing people's darker impulses and giving them new murderous abilities would have been a game-changer in Thanos' hands. If it happened to a fully grown Thanos, it might have even been strong enough to really turn the tide of the battle. But thanks to some quick thinking by Thor and his father, the Avengers are able to defeat a foe who likely could have wiped out entire planets.