I wrote this a while back but forgot to post it so here it is now. Not a crossover thought it was inspired by a few thing including, obviously, Lovecraft.

Also, Coulson is alive just 'cause.

Not beta read so sorry about any errors

It should have been easy.
The Avengers was assembled to deal with a bunch of crazy cultists. It was really only Stark and Banner that was needed, and that was only because of strange energy emissions that bore some similarities to the tesseract. But when it was discovered that they had hostages the rest of the team decided to come help.
They rescued the hostages in short order and it looked like everything was fine, until the lead cultist shouted an order and the others stabbed themselves with their own sacrificial knifes.
There was a seconded or too of shock silence at this waste of life before the lights went out and everything when insane.

Clint had noticed the symbols on the floor of the room when they had entered but they hadn't seamed very important. Now, though, they were glowing, illuminating the room after all the other lights were extinguished. They were also moving. He still didn't know what they meant but they certainly seamed important now.
If you looked at them too long they seemed to lead the eye to the corner of the room where-

don't look

There were also a number of … cables that had appeared when the lights went out. All different sizes, they stretched around the room, binding the Avengers in place. They all seemed connected to the coroner of the room where -

don'tlookdon'tlookdon'tlook

Natasha was staring straight ahead, gun out and aiming, stock still. Too still. She didn't even seem to be breathing. The only movement was from the thin trickles of blood, slowly dripping from her eyes and mouth. She was the only one looking at the thing in the corner-

don'tlookdon'tlookdon'tlook

Ironman was flat on his back. JARVIS had shut down with a loud screech all of them heard when the lights shut off. The glow of his arc reactor still bright as if the thick tendrils covering the Avengers couldn't touch it. But every second there seemed to be more of them, getting closer and closer to it.
He could be heard yelling threats and obscenities, but without the suits amplification it was muffled. All he could see was the ceiling and he kept demanding to know what was going on. But no one could answer him.

Captain America was half crouched half kneeling behind his shield. His normally strong voice seemed thin and reedy as he struggled though half whispered prayers.

The Hulk, who had bust out of Banner with no warning when the lights went out was wrapped loosely in the vines. It was possible he could have torn free if it wasn't for the fact that he was currently rocking back and forth, whimpering.

Thor was wrapped tightly. His struggles weak and feeble. The fact that he was still fighting would have been more encouraging if it wasn't for the fact that his face showed nothing but agony, silently screaming, eyes wild and unseeing.
Mjolnir was too was wrapped in the- oh just called them what they are- tentacles. Some of them seeming to be going though the enchanted weapon.

On the floor were the bodes of the cultists and dancing around them was the leader. Crying and laughing he shouted words of some unknown, guttural, language in-between raving and worshipping the thing in the corner.

don'tlookdon'tlookdon'tlook!

Clint may not have a super power but he did have perfect aim. And along with that came great vision. Part of it was natural talent, part of it was years of dedicated training. He noticed things, other then shooting things, that was his best skill.
Never before had he spent so much effort in trying not to see something.

He didn't know what was in the corner but he knew, knew, if he looked it was over. It would have him and he would never, ever, get away.
It was there, and there was only so long he look away. Closing his eyes didn't help. It was as if he was been pulled towards it and whenever time he opened his eyes again he was closer to it-

don't LOOK!

There was a sound of a door opening.

Finally, some light that didn't come from the glowing runes scrawled across the floor and walls entered the room.

The light was blocked momentarily as Agent Coulson walked calmly into the room.

Coulson was wearing his 'Spook' suit. The one he wore when he wanted to be unmistakably a government agent. Pitch black with a stark white shirt he looked like someone out of the MIB. He even had one of those obvious ear pieces the FBI wear. Also, despite the dim light, he was wearing dark sunglasses which he made no move to take off.

"What do you think you're doing?"

The cult leader stopped his raving and turned to Coulson.
"Fall Down and Worship The Dark One! The Great Best Who Will Devourer Us All! The Crawling Chaos that Feasts-"
"I don't believe I was addressing you" Coulson cut him off.
And then he turned to the thing in the corner, "Well?"

For the first time Clint didn't feel that awful urge to look at the thing in the corner. The feeling of creeping dread was replaced by utter incredulous.

Then some of the tentacles started to move. They slithered over and around and then into the bodies of the fallen cult members. The bodies that then started to move. Blood still dipping from self inflicted wounds the corpses jerked upright like macabre puppets.
As one, they lifted there heads and spoke.

"We were summoned"

"And do you have the paper work for all this?"

"We have flayed the skin of a thousand of our offspring. We have slain our parent and used it's bones -"

"Yes, yes, that's all well and good. You do seem to have followed correct procedure" Coulson whipped out a tablet from somewhere. An actual tablet made of stone. Strange runes were craved on it.
"Unfortunately it seems that while you did indeed use the correct materials you have neglected to fill it in correctly. Transdimensional Migration forms exist for a reason. Failure to correctly complete your paperwork means you are subject to heavy fines, loss of limbs and possibly the right to exist."
The cult leader raised his knife threateningly.
"Not to mention what obstructing a duly appointed Agent in the course of their duty can result in."
The cult leader lowered his knife and looked back at the thing in the corner.

"We were summoned"
The multiple voices, while still horrific and reinstating though the bones of all who heard them, now also seem somehow plaintive.

"Incorrectly. You never filled in a point of arrival. The rest of this form isn't much better but at lest it's filled in. Though your worshipper/minion procurement section seams unnecessarily vague" He looked around at the dangling bodies. "How did you even get these guys?"

"We sent out dreams of terrors and horrors. Full of forbidden knowledge and secret power"

"Ah, a traditionalist. And you didn't think to ask any of them were they lived?"

Tentacles started creeping up the only living cultist as he started jerking violently. His knife flailed wildly until it suddenly buried itself in his own chest. Screaming in agony the lead cultist reached into the cavity and tore out his heart. Held up by the tentacles he used the last of his strength to thrust the heart at Coulson.

"Bribery isn't going to help your case. You already have a misfiled-transmigration report and a obstruction and misalignment form to fill in, don't make me add a disciplinary report as well."

"We were summoned" Now the voices seemed positively sulky.

Coulson sighed.
"Look, you seem like a decent eldritch being. And I can see this is your first time. Since I don't particularly want to fill out the incident report for this myself why don't you just clean up here, go back home, lay low for a century or two and reapply. Fill everything out correctly this time and I'm sure you'll be back in a few millennia.

The voices made a horrible screeching sound and the thing in corner started to move. All the tentacles started to writhe and the glowing ruins started to pulse. A horrible indescribable noise added to the voices, volume climbing until it seemed to fill the whole world.

And then suddenly silence.

Fluorescent lights overhead flickered on.

The room was empty.
The Avengers were alone with Agent Coulson.
There were no symbols scrawled over the walls and floor. No blood smeared on dripping. No tentacles No bodies. No cultists.

There was a feeling of a lifted oppression. Like everyone had just woke up from a horrible dream. And, like a dream, the details were beginning to get fuzzy.
Tony's armour fell apart, the emergency escape protocol finally kicking in.
"Ok. What the hell just happened?!"

"I didn't always work for Shield." Coulson carefully removed his earpiece, blood dripped down staining his nice white shirt.
"I used to work for the Infernal Revenue Service."
"Infer- wait. The IRS!"
"An incredibly top-secret section of the IRS, yes."
"As much as I want to say I always knew the IRS was hinky, the IRS? really?"
"It's a long story. Lets just say there are curtain...beings that ...live in a dimension that is in no way like our own. Occasionally they like to try and get though to different plans of existence but to do that they need to follow certain rules. And because these beings are pure chaos we do, to some extent, get to set these rules. Where it used to be complex rituals and secret incantations nowadays it's more like lots and lots of red tape. Of course some things can't be changed so the red is usually blood"
"So it's the IRS because.."
"Because no one likes the IRS. That type of wide spread annoyance to downright fear is incredibly useful. A lot of what we need to use isn't actually physical, or even really real in any meaningful way. But it works. Thankfully no one really fully retires form the IRS so I had a few things on hand when I realised what was happening."
Tony looked like he was ready to explode in a million unanswered questions so Clint decided to interrupt.
"So what now?"
"Now I'm going to fill out an after action report and then collapse into bed. I recommend you all do the same. Don't worry about the screaming night terrors, that's fairly standard. They'll go away pretty quickly and you'll probably forget all the details of what happened, if not the entire encounter."
"Forget?!" Tony squawked. "You can make us forget?"
"I don't need to. The human mind has a tremendous ability to protect itself. No one is really meant to stare into the void like that. Not while staying sane anyway."
He looked around at the other Avenger who were all in various stages of semi to unconsciousness.
"Time to go home"

Clint took a look at the now empty corner and shivered.
"Fine by me."