Set in Supernatural season 7, somewhere close after Bobby's death (and before the brothers realize he's still there) and in the middle of the battle of Sokovia.
This is my first crossover between those two and my first Supernatural-fic in a long while… I hope you like it. Reviews are always welcome.

Now

Those hellhounds were fast. Pietro doubled over in the ruins of what had only minutes before been a kitchen, trying to catch his breath. Not that he would admit it in front of the gloomy archer but they were almost as fast as him. And they had something playing for them, something Pietro had been running out of for the past year: time. They would get him. Tear him apart. Drag him to hell.
As if he hadn't been there for most of his life.

Not even Ultron saw them coming, not even Wanda. They were here for him only.
Hot spittle splattered the right side of his face, and Pietro ran again. He knew he would not live to see how the battle of Sokovia ended. But he would take as many of Ultron's soldiers with him as he could.


One year ago

It was three o'clock in the morning when the damn cell started to ring. Eyes barely opened, Dean rummaged through his bag, shoving away empty bottles and dirty clothes – and the dagger he'd forgotten to put out after slicing a skin changer. "Son of a bitch!" Sucking at the bloody cut on his finger, he finally realized his mistake.
It wasn't his phone beeping.
Sam, woken up or not having slept at all, reached for the jacket hanging on the only chair in the room.
Bobby's jacket.
"Ma…Maximoff?" Sam squinted as he read. "Name ring a bell?"
Dean shook his head.
"I guess there's a lot we'll never know." Sam's grief was apparent on his face, illuminated by the bluish glow of the display.
Dean nodded. "I guess." His fingers dove through his hair. "For heaven's sake, just pick up!"
"Okay, okay… Hello?"
"Mr. Singer?" The voice at the other end was young, male and thick with an accent Sam couldn't place.
"Uhm… no, I am Sam Winchester. We're…" Sam swallowed. "We were friends of Bobby. Mr. Singer."
"I need him. Personally. Now."
Sam sighed. "I'm afraid that's not possible."
"Then make it possible."
"I can't."
"My father promised this man will help us." The man on the phone seemed to become angrier with every second, and more anxious. A dangerous mixture… and just the cocktail that was flowing through the Winchester's veins. Sam opened his mouth but the other voice kept on, almost stumbling over the words: "He owes my family. My name is Pietro Maximoff, my father's name was Maxim. He owes us. Get me Mr. Singer!"
"I can't, okay?" Sam's voice was raw with exasperation. "He is dead!"
For a moment, there was silence. Then Pietro's voice reappeared, broken by tears. "That is not possible."
"I'm afraid it is." Sam took a deep breath, calmed by the other's obvious distress. He must have known Bobby to react like this. "I'm sorry."
"It can't be."
"Maybe we can help", Sam offered, "we are… in the same business as Bobby." He shrugged at Dean. "What did you want from him?"
A cracked laughter wriggle through the phone. "Just a way to outsmart death."
"Death? You mean…" Finally acknowledging Dean's gestures, Sam put the caller on loudspeaker.
"I mean I need to stop it." The pain in Pietro's voice was palpable. "Please. Our father said if there was something between earth and heaven that we could not deal with, Mr. Singer would help us. I need this help right now. I can't lose her."
Dean shook his head, mouthing "no". Deals with death were not a solution. He did not regret the deal he'd made himself years ago – but that had been different. He'd done it to save Sam. He'd do everything to save Sam. He was all he had left, damn it. Now with Bobby gone more than ever.
But that didn't mean it was a generally good idea. Especially not with Crowley running the business now.
Sam sighed. "I'm sorry. Trust me, I know how it feels to lose someone, but…"
"I don't care how you feel! I don't care what it costs. You know a way. Tell me. Please", Pietro's voice broke again. "She is all I have."
"What, your girlfriend?" Dean rolled his eyes.
"My sister. She is sick, the doctors say she will die tonight. I can't let that happen, I promised I would protect her. Always."

All I have. Dean swallowed. The memories he had just pushed back returned, the loss as overwhelming as in the night he'd sat next to his little brother's corpse, pleading with Sam's ghost, with destiny, with god…

"Dean! No." Sam shook his head in warning but Dean ignored him. "What would you do to save her?"
"Everything." The answer came without hesitation.
"Sell your soul? Like, for real?"
Pietro laughed sadly. "I already did. But if I have to do it again, yes."
"Dean", Sam hissed, "you just said we couldn't…" Dean shook his head. "You can make a deal, Maximoff. You can save her. But you will suffer for it. In a way you… trust me, you can't even imagine."
"Tell me."
Sam backed away, old and new guilt seeping through him as Dean told Pietro Maximoff how to sell his soul… and to protect the most important thing in his life.


Now

When the helicarrier arrived, Pietro could barely keep on his feet. Just one more moment.
Crowley had promised him to save Wanda, and he had kept his promise. In a matter of minutes his sister had been healed, a miracle to even the greatest of HYDRA's scientists.
Her life, and one year for him to live. That was the deal they had made, and Pietro was almost ready. There was just one more thing to do. He needed someone to look out for his sister when he was gone. Someone who made sure she kept on living without him.

Suddenly he saw Clint running back to the ruins. Pietro smiled. Save the two of them.
The dogs had encircled him and the archer. "Out of breath, little boy?" Crowley's voice filled the air, smug as it had been one year ago. "Can't you just stop running and try to summon up a little courage? I thought you were a warrior. Or a… how you do like to call yourselves? Avenger? You're coward, boy. Just like the rest."

One of the hellhounds tore at his right leg. Just one moment.

Ultron's fighters had closed in on them as well, loading guns that unfortunately would not harm the demonic predators. But they would help protecting his sister.
When the attack started, Pietro gathered every strength he had and jumped into the line of fire.

Clint stared at him.
The hellhounds barked.
The bullets hit him.
Pietro grinned, blood running out of his mouth, his legs, his chest. The look in Clint's eyes told him his plan had worked. The archer would take care of Wanda.

He winked at the demon. "You didn't see that coming."
Crowley shook his head, astonished for only a second. Then he nodded, and the hounds dragged Pietro to hell.