Here's the prologue for a new story I thought of ages ago, inspired in parts by Iris Potter: Out for Revenge (unfortunately I forgot who wrote it) and Child of the Storm by Nimbus Lleweyn. You should go read them.
Enjoy this crossover!

-Wolf

The day everything changed…

"Foolish boy! Move!" Voldemort yelled at the man standing between him and his target. The doorway to the Potter Residence might not be the best spot for a murder – but what is necessary must be done.
"Lily! Take the children and go!" he yelled, and Voldemort remembered what he had been told by the mystery voice in his head: don't kill the parents, only incapacitate them.
Stupefy, he thought, and the red burst of light connected with Potter's chest, causing him to fall unconscious.

He proceeded through the living room and up the stairs, and found the right room.

Inside, a woman stood in front of two cots: one with a baby Harry Potter, one with a two-year-old Christine Potter. He had been told by his master – the only being in this universe that was stronger than him – to kill the boy, but he could see that the woman was shielding him more than the girl, meaning that the consequences of him killing her were less than that from killing the boy, but he was his target.
Snap out of it, Tom! He scolded himself, Kill the boy, and you'll be preventing the prophecy! You will rule over muggles and wizards alike for eternity! Stupefy! Again, the red bolt of light hit the woman's chest as she was crying "Not Harry, please not Harry, anyone but Harry!"

He raised his wand a third time, and spoke the incantation for his signature spell: "Avada Kedavra!"

He expected them to die, like every other child he'd killed.
Except they didn't.
Instead, a golden-green barrier shot up between himself and the two young wizards. He knew this wasn't part of normal magic – magical barriers were normally a transparent blue.

There was another flash of golden-green light, and a man appeared. He had long, silky, black hair and emerald green eyes. The clothes he wore were not of Earth, but of Asgard.
And, in this moment, he knew he had fucked up, for this was Loki, God of Mischief, Trickery, and a skilled user of magic from both Midgard and Asgard alike.

The god glared at Voldemort. "You dare come into this household, and attempt to kill my daughter?" he snarled, and the faint scars on his lips from what the humans believed were visible for a split-second while he spoke.
"I – I didn't know, my Lord." He said – it was rarely that Lord Voldemort himself referred to someone as my Lord, but Loki was a special case.
"That leaves no room for error." His lips were contorted into a horrible sneer that left Voldemort wondering why he had done this. "I thought you were going to become one of my greatest followers, but this, this?!"
"I'm sorry –"
"SORRY DOESN'T CUT IT!" Loki yelled at him, "I hope you have a backup plan, Riddle, because you've just messed with something you shouldn't have. You have ten seconds." Loki's irises slowly turned from green to red, as Voldemort lifted his wand again, in one last attempt, and screamed "Avada Kedavra!"
Loki moved his hands to contort his barrier around himself and the children. There were a few holes in the energy field, however, causing Christine and Harry to gain some scars that connect them to the mind of the Dark Lord himself. Loki, at this time, didn't know that, and simply banished Voldemort's consciousness from his body, causing said body to dissolve into dust.

And then the god disappeared, not leaving any trace of him even being there, other than two things for his daughter: an enchantment to protect her from magical means of attack that would wear thin over time and eventually disappear altogether by the time she was twelve, and a birthday gift (her birthday was October 31st, after all): a necklace that would boost her magic and protect her from dementors and every type of venom, including that of the basilisk and Asgardian Viper – the most potent poisons in the Nine Realms. It would only be able to be removed under her own free will.

After he was gone, Lily came to, as did James – who rushed up the stairs with no regard for his personal safety (they were busted and multiple were missing). When they saw that both children were fine, they embraced them both and each other.

Albus Dumbledore soon appeared in the doorway. "James, Lily." He said, and they turned. "Perhaps it would be best if you let the Boy-Who-Lived and his half-sister breath?" both parents were confused at the use of the Boy-Who-Lived and James was even more so about the part about his half-sister.
"What do you mean?" Lily asked.
"Dear Harry here defeated the Dark Lord, can't you tell?" he lied. It came to him easily, lying did. "And, James, Christine here isn't your daughter – she was Lily's by another man." That part he knew was true.
James stared at Lily in disbelief. "You cheated?"
"It wasn't my fault!" she protested. "He looked exactly like you did, and it was only the morning after that I realised! I swear!" from the look on James' face, she could tell he thought she had lied.
"It's obvious she's lying, isn't it, James?" Dumbledore said, enforcing what James had thought, "It might be best to send Christine to live with her muggle relatives, so she doesn't feel left out or jealous of Harry's fame."
"No!" Lily blurted out the same time that James said "It's for the best."
"I see… luckily, I brought just the thing." Dumbledore fished around in his pocket for a few seconds before he pulled out a small pouch. He handed it to James. "Just slip those on her wrists, and she'll never lie, cheat, or disagree again!" his tone was cheerful, and he knew that his plan was going perfectly.

And James took the two bracelets out of the bag. Lily recognised them, and screamed. Dumbledore cast a full body-bind curse on her, and James slid the Will Cuffs onto Lily's wrists, and her expression became blank. Dumbledore released the full body-bind.
"Yes, we will send Christine away." Lily said, a blank expression on her face as she handed the two-year-old Christine over to Dumbledore, who chuckled and said "You did the right thing, James," as he left with Christine, who was dropped off at the Dursleys' house on his way back to Hogwarts.