So this part of the story takes place right during the movie Thor and during my fic Freed.
The only thing that made living in the royal stables tolerable were the stable hands and servants. They weren't particularly nice, per say, but they were terrible gossips. If some lord in the castle knows it, then a servant knows it as well. It's an unspoken law of the Allfather's high halls. If one servant knows something, it's a guarantee they're going to share it with another. That's how Sleipnir was one of the first to know that Thor had been banished, the Allfather had fallen into Odinsleep, and that Loki was currently ruling as an interim king.
Sleipnir had always been closer to his siblings than his father. When he was young, he had no idea why Loki always seemed sad when Sleipnir was there. It wasn't until he was older that his sister told him: he was a child of rape. When he learned what that was, he was horrified. Sleipnir had become determined to look less Svaðilfari, his biological father. Hela and Fenrir had helped him hone his mediocre shapeshifting skills until he was able to take human form. He was tall, nearly seven feet, with a large frame that was more muscles than anything else. The had dark tanned skin, with a long face, green eyes, and shaggy black hair. He was even taller than his uncle like this. Sleipnir hated this form.
When Sleipnir heard the news, he realised it was time to execute his escape plan. A while back, he had stolen some of the stable boys' clothes and now all he needed was a distraction. At nightfall he would turn into his human form; change into the clothes; release the other horses, and in the chaos try to escape. It wasn't the best plan, but Sleipnir couldn't say he was the cleverest of Loki's children. That would be Jörmungandr, who lived up to the reputation snakes have: cold, clever, and shrewd. After his escape, he would head to the one place nobody ever went. Helheim.
Night fell, and he went through with his plan. He was easily able to unlock his stall, and Sleipnir snuck out into the hallway. From there, he was easily able to open the doors to let all the other horses out. He had spoken to them before, and everyone knew their part. The various horses thundered outside, and Sleipnir knew he would have a few moments where the stable hands would think he was one of them. He started casting the spell that would allow him to access the world tree. Once he was out there, his speed would make him quite unnoticeable. He suddenly was cast in darkness, one tiny horse in the shadow of the World Tree. The idea of 'nine realms' was an outdated concept, but Asgard continued using it. Honestly, the realms were… like fruit, hanging from the branches of Yggdrasil. The nine realms were the closest to the trunk and the largest. There were others, farther away, but the Bifrost doesn't go that far.
When he arrived in Helheim, the place was exactly as he last remembered. Everything was dark, gray, and stagnant. Sleipnir trotted across some symbolic river and into the hall in front of him. All around him there were pale shades of the dead. They always disturbed him, with their blank gazes and hopeless looks. When he entered his sister's hall, he saw the high arched ceiling, the tall windows, and on the throne, his sister looking miserable.
"Hela!" She turned at the sound of his voice, and brightened. Now he could see her whole face. Her right side had beautiful porcelain skin, with a half green half brown eye and dark hair. Her left, however, had dark skin pulled too thin over her face, a cloudy blue eye, and brittle hair. Most people thought she was scary, but to Sleipnir, his sister would always just be.
"Sleipnir, so good to see you!" She scrunched up her nose. "Wait. Does this mean Odin is coming as well?"
"Nope! I escaped! And…" He shuffled nervously, well, as nervously as a horse could shuffle. "I was wondering if you had any way to hide me?"
She smiled, showing sharp white teeth. "I… I think so. I've been meaning to use this spell on myself. It can turn any being mortal. I had plans to hide myself when Odin wasn't looking. It'll turn me into a Midgardian at, well, really any point in their history. I still haven't worked that part out. But if we do it, I can make it so our souls are connected. We'll be close, siblings, childhood best friends, something like that."
"Please tell me not soulmates?" Sleipnir grimaced at the thought.
She chuckled. "I can't guarantee anything. We're going to have to hope. There's also the chance we might end up different genders."
"I don't want to be a female! They have bleeding and children." Sleipnir shuddered, and Hela giggled at her little brother again.
"I have it set so that once we turn thirty, our powers will manifest. They might show up as well on the off chance that we're under some sort of magical attack. You got all that?"
"Yeah. Let's do it."
On February 17th, 1989, Pietro and Wanda Maximoff were born. The twins both had dark hair and eyes, and had no idea of their own true power.