WARNING: This is pure, unedited wish fullfilment. Beware of typos and shitty grammar. Also, I am 100% guaranteed to get at least one character's lore/personality wrong, so YMMV. Enjoy!
Matt Murdock, formerly known as Daredevil, snuck his way to the edge of a cliff. He may have been dead, but this mission was for the sake of an old friend.
"Hey guys, I found Mephisto's castle, so if we're gonna do this, we gotta do it now." He whispered, his voice magically transmitted back to Earth. Suddenly, a massive glowing energy portal appeared behind him, and an army stepped out.
Doctor Stange, Scarlett Witch, Iron Man, Thor, Captain America, the Hulk, the Fantastic Four, the X-Men, including a very angry Wolverine, the Punisher, Ant-Man, War Machine, Deadpool, Squirrel Girl, both Captain and Ms. Marvel, the Guardians of the Galaxy all entered Hell. Every single superhero on and beyond Earth had come to the underworld for one purpose, to save their friend. Even some of their foes, such as Doctor Doom and Kraven the Hunter had joined them.
Two decades ago, Mephisto, a Lord of Hell had tricked the hero Spider-Man into making a deal that ruined the hero's life. The army that was now assembled on Mephisto's front lawn had come to set things right.
Captain America, because who else could lead them, stepped forward and raised his mighty shield, which had struck down despots, villains and terrorists. Soon, he would add a Devil to that list.
That Devil watched all of this unfold from atop his throne. He chuckled.
"You want Peter Parker's soul? Very well. Come and get it." He laughed, his mirth sounding like a thousand nails on a thousand chalkboards.
Mephisto raised his hand, and an army of trillions of demons, all twisted mockeries of mortal's worst nightmares surrounded his castle.
"AVENGERS ASSEMBLE! FOR SPIDER-MAN!" Cap bellowed, his mighty shout shaking the very foundations of Hell. The army of heroes behind him screamed out war cries before charging at the gates of Mephisto's castle.
"Hit it Mick!" Deadpool joked as he drew his favorite twin pistols. For years, Spider-Man had been the only hero who'd ever treated him as anything other than a nuisance. Today, Wade would finally pay back his friend's kindness.
Deapool leaped over the Punisher, who had begun firing an LMG into the horde of demons. he fired point blank at a demon the size of a small child, knocking the monster's head clean off. Deadpool whipped his other hand around and fired at another, and another. Eventually, his guns clicked empty, and Deadpool abandoned them, unsheathing his katanas and transforming into a whirling blender of death.
"HAVE AT THEE, DEMONS!" Thor bellowed, his eyes burning with power. The God of Thunder, Lord of Asgard and Protector of The Nine Realms raised his hammer and for the first time in a millenia, rain began to fall on the plains of Hell as the Thunderer conjured a massive storm more powerful than any that had raged on Earth. Lighting crashed down, instantly vaporizing demons left and right.
Meanwhile, the Thing of the Fantastic Four noticed one of their allies.
"Doomie? What the hell are you doing here?" Ben slammed his massive, rock like fist into a particularly large demon, instantly crushing it's head into pulp.
"Spider-Man is one of the most noble foes I have ever had the honor of fighting." Doom replied. "Doom would see his rightfull happiness restored." Doom raised his hand, sending a massive charge of magical energy at the demonic horde, vaporizing them all.
Rocket Racoon clung to the shoulder of his fellow Guardian Star-Lord, the two of them blasting away at the oncoming horde. "Are you sure about this Pete? I mean, I like the webhead too, but unleashing Groot in Hell is a big gamble!"
"I am. This is important Rocky. No one deserves what Spider-Man had to go through. Let the big guy loose." Star-Lord replied.
Rocket handed Quill the massive blaster he had brought to this assault, and pulled a twig out of his belt pouch. "Alright, but if we Grootify all of Hell I'm blaming you."
Rocket chucked the twig as far as his tiny arm could. He couldn't see where it landed, but it must've hit dirt somewhere, because a massive humanoid tree sprung up in seconds in the middle of the demonic horde.
"I AM GROOT!"
The massive Floral Collossi bellowed, trillions of branches snaking out to pierce and strangle demons, and the tree only kept growing.
Grootfall had come to Hell.
A massive demon lumbered forward to oppose Groot. "Iron Man, incoming!" Clint Barton, the original Hawkeye shouted. The Armored Avenger rocketed forward.
"B.O.S.S, reroute all auxiliary power to the chest unit," Tony ordered. "Complete," his armor's operating system replied.
Iron Man flew right up to the massive demon and threw his arms back, unleashing a massive blast of repulsor energy right into it's chest, carving a hole the size of an aircraft carrier all the way through the demon. Iron Man fell, his suit struggling to maintain power after the blast he had unleashed. Luckily, War Machine arrived and caught him, allowing the Model 70 to compensate.
With Groot now on the field, the heroes broke through Mephisto's lines, their combined power easily breaking through the mere stone of the Devil's castle wall. Ant-Man expanded his frame to giant size, and threw Luke Cage, sending the invulnerable Hero For Hire crashing into Mephisto's front gate.
Daredevil was the next hero to enter the Devil's fortress. "I'm surprised you're here, Luke. T'challa or Emma Frost offer to foot your bill?"
Cage shook his head. "Nah, I'm saving Spidey's butt pro bono. Man deserves it."
"Agreed."
Both heroes charged deeper into the fortress, yelling at the top of their lungs.
The Immortal Hulk chose not to follow the other heroes through the gate. "Hey Logan," He jerked his head at Wolverine. "Hop on."
The X-Man leaped onto the Hulk's back, and the two launched into the air, crashing directly into Mephisto's throne room.
"Hey. You did something really bad to a really good friend of ours. We're gonna make you hurt for that." The Hulk leaped at Mephisto, knocking the Devil off his throne with a mighty blow that shook the heavens.
Wolverine, not to be left out, swung his arm, slashing three massive gouges in Mephisto's chest. "Yeah, welcome to a world of pain, Bub."
Mephisto attempted to repel his attackers with magic, but found himself unable to call upon his powers. "What!?" He looked to his left to find the Scarlet Witch, her hands casting the hex that cut off his powers. Wanda's face hid unimaginable fury.
"Hello, Mephisto. I heard you tried to tear apart someone else's family."
Suddenly, the Devil's body contorted with pain. Mephisto screamed, unaccustomed to agony.
Meanwhile, the other heroes fought their way to the Devil's throne room. Captain America, having gained the lead, hurled his shield at the door, shattering it.
Mephisto regained access to barely enough power to stop the Witch from tearing him apart, but was prevented from gaining the momentum by Cap, who blocked his next magical blast. This gave the Hulk another opening to strike, sending the Devil crashing to the ground.
Mephisto coughed, spitting up blood for the first time in millenia. "Why? What possible reason do you meddlesome mortals have to invade my home?"
Cap answered. "This is for Peter Parker you demonic son of a bitch." Cap slammed his fist into Mephisto's jaw, knocking the Devil back onto the ground.
"Alright! I'll offer you a new deal! I'll give you his happiness, and in exchange, none of you may ever set foot in my domain ever again!"
Mephisto tried to raise his hand so Cap could shake it, but Steve ignored it. "Good." Cap turned and started walking away. "We did it guys. Let's go home."
Back on Earth, in New York City, Mary Jane Watson-Parker opened the door to her husband's hospital room to allow nearly a hundred superheroes to crowd into the tiny space.
"Hey y'all. What'd I miss?" Peter Parker joked.