I'm not sure if there are many fanfictions specifically with that plot. But you could try searching on big fanfiction platforms like Archive of Our Own. They have a vast collection, and you might be able to find something that comes close to this concept.
Perhaps there's a prophecy involved that foretells either Fred or Hermione's death, and Hermione, in her cleverness, figures out a way to twist it so that she takes Fred's place. The plot might then revolve around the lead - up to this moment, with their relationship growing and changing. We could see flashbacks of their time together at Hogwarts, how they became friends or more than friends. And then, when the moment of sacrifice comes, it's a very emotional and heart - wrenching scene.
Well, it would be a huge shift. The Golden Trio would be broken. Harry and Ron would be devastated. Hermione's intelligence and problem - solving skills are crucial for their fight against Voldemort. Her death might lead to a more desperate and chaotic situation for the remaining characters. They might lose some of their strategic edge and it could also cause a lot of emotional turmoil for the entire wizarding community who saw Hermione as a symbol of hope for Muggle - borns.
Harry would be crushed. He'd feel like he lost a sister. Ron would be in total despair, probably blaming himself. Ginny would be heartbroken too as Hermione was like a sister to her. The Weasley family, who had accepted Hermione as one of their own, would be in mourning. They'd all wonder how they could have prevented it and would be angry at the unfairness of it all.
Fred could use his knowledge of the Weasley family's magic tricks and inventions. He might have some secret gadget or spell that can break through the barriers or spells that are being used to torture Hermione.
One possible fanfiction could be where Fred and Hermione discover their Animagus forms together. Maybe they transform in the Forbidden Forest and have to rely on each other to get back to Hogwarts safely. Their forms could be something that represents their personalities, like Fred could be a fox, known for its cleverness and playfulness, and Hermione could be an owl, symbolizing wisdom.
Well, in some versions, Fred might use his quick-witted magic skills. He could cast a distraction spell to draw Bellatrix's attention away from Hermione. Maybe a spell that creates multiple illusions of himself, confusing Bellatrix long enough for him to grab Hermione and disapparate them to safety.
One reason could be that they played a particularly mischievous prank on her that went too far. For example, if they made her late for an important exam or meeting with a professor by using one of their wild magical inventions. Hermione, being a stickler for rules and academic achievement, might lose her temper and hex them in retaliation.
I'm not entirely sure if there is a specific 'Fred terrified of Hermione' fanfiction. It depends on the creativity of the fanfic writers in the fandom. Some writers might explore this unique dynamic for comedic effect, while others might use it to create a more complex and unusual plot. However, the best way to know is to search for it as I mentioned before.
Well, it's a wild fanfiction concept. Fanfic writers might play with the idea by having some sort of magical mix - up or a different relationship path for Hermione. Maybe there's a spell gone wrong that leads to this unexpected pregnancy. But it would surely be a huge departure from the original story's themes and relationships.