The Avengers had faced countless enemies, from alien warlords to rampaging monsters, but this was different.

A S.H.I.E.L.D. alert brought them to an abandoned research facility in upstate New York. The skies were a dull gray as the Quinjet touched down, the air thick with unease. Captain America led the team out, his shield at the ready, flanked by Iron Man, Thor, Black Widow, and Hawkeye.

"What's the situation, Stark?" Cap asked, scanning the desolate surroundings.

Tony Stark's faceplate lifted as he checked the readouts on his suit. "Energy signatures are off the charts. Whatever's in there is playing havoc with my sensors."

"This place reeks of dark magic," Thor muttered, gripping Mjolnir tightly. "I sense an ancient presence."

They ventured into the facility, their footsteps echoing in the eerily silent corridors. The deeper they went, the more unsettling the environment became. The walls seemed to bleed with some dark, oily substance, and an oppressive chill filled the air.

"I don't like this," Natasha whispered, her keen senses on high alert. "Something's wrong."

Suddenly, a low, guttural scraping echoed from down the hall, followed by a wet sloshing sound. The Avengers froze. From the shadows, a figure emerged. Its rotting, decayed form was barely human, dressed in tattered, moldy clothes. Its eyes gleamed with malice, an unnerving grin stretching across its face.

"What the hell is that?" Hawkeye muttered, arrow nocked.

"It's not human," Tony said, analyzing the creature. "I'm getting no vitals."

Before anyone could react, the figure lunged. It moved faster than anything its decrepit appearance suggested, its hand plunging through the wall and grabbing Thor by the arm. The God of Thunder grunted, caught off guard as the creature dragged him into the wall, pulling him into a black, goo-like void.

"Thor!" Steve shouted.

The team scrambled into action. Cap threw his shield, only for it to pass through the creature and clatter uselessly to the ground. Natasha fired at it, but her bullets seemed to dissolve before reaching it. Even Tony's repulsor beams fizzled out as the darkness surrounding the figure absorbed the energy.

"Get out of the wall! Quickly!" Iron Man yelled, his sensors screaming warnings of unstable spatial anomalies. But Thor was gone.

"This thing—" Tony started, scanning rapidly. "It's warping reality. I've never seen anything like it. It must be some kind of breach."

Steve gritted his teeth. "We don't leave anyone behind."

Suddenly, a hand shot out of the wall, this time aiming for Steve. Black Widow lunged, pushing Cap out of the way, but she wasn't fast enough. The creature's slimy fingers grazed her, leaving behind a thick, black residue. Her face twisted in pain as the substance started to eat away at her suit.

Tony blasted the wall, creating a hole where the creature had disappeared. But the void it left behind was chilling—an inky blackness that seemed to swallow everything in its path.

"This thing isn't like anything we've faced," Tony said, panic creeping into his voice. "It's not a person, or even an alien. It's something else entirely."

Steve knelt beside Natasha, who gritted her teeth in pain. "What is it? What's happening to her?"

Tony scanned the residue, frowning deeply. "It's corrosive… like it's dissolving her at a molecular level."

Suddenly, Thor burst out of the void, gasping for breath. His armor was shredded, and his skin was covered in the same dark ooze. But the God of Thunder roared with defiance, hurling Mjolnir at the creature as it reappeared. The hammer slammed into it, causing a shockwave that sent it stumbling back into the shadows.

"It was trying to drag me into its dimension," Thor growled. "A place of darkness and decay."

"That's SCP-106," a voice interrupted from behind them. The Avengers turned to see a group of heavily armored S.H.I.E.L.D. operatives rushing in, led by Director Maria Hill. "It's not something you can fight. It's an anomaly—older than humanity. We call it 'The Old Man.' It can phase through solid matter and corrupt anything it touches. Conventional weapons won't work."

"What do you suggest then?" Steve asked, frustration evident.

Hill pointed to a high-tech containment device being wheeled in by the agents. "We lure it into the trap. It's worked before… temporarily. But you need to distract it long enough."

Thor, battered but unbroken, lifted his hammer again. "I'll buy you time."

As the operatives set up the containment field, Thor charged forward, drawing SCP-106's attention. The creature hissed, its decayed form flickering in and out of reality as it lunged for the Avenger. Thor met it head-on, his hammer crackling with lightning, while Iron Man and Hawkeye provided cover from a distance.

The containment device hummed to life, its energy field growing stronger by the second. Thor gave a final shout, hurling Mjolnir with all his strength. The hammer collided with SCP-106, pushing it backward into the energy field. The containment device snapped shut, sealing the creature inside.

The facility fell silent.

"You think that'll hold it?" Steve asked, panting from exertion.

"For now," Hill said, wiping sweat from her brow. "But it always finds a way out. We need to move fast."

Thor collapsed to one knee, exhausted. "That was no mortal foe. What in all the realms was that?"

Hill shook her head. "An SCP anomaly. One of many we barely understand. But trust me—this one is better left locked away."

The Avengers regrouped, knowing full well that this battle was far from over.