Authors need to first understand the core elements of both universes. For Harry Potter, it's the magic, Hogwarts, and the fight against Voldemort. In KHR, it's the Mafia families and the growth of Tsuna. Then, they can start by introducing characters from one universe into the other in a believable way. For instance, if Harry were to enter the KHR world, how would he adapt to the different rules and powers there?
Finally, respecting the original tones of both series is crucial. Harry Potter has a more traditional fantasy - adventure tone with a focus on good vs evil. KHR has a mix of comedy, action, and coming - of - age elements. A good fanfiction will find a way to incorporate both tones, perhaps having some light - hearted moments between the more serious battles, just like in the original series.
Well, one way is through the use of magic portals. For instance, a portal could open between the two universes and Black Widow accidentally steps through it into the Harry Potter world. Or vice versa, a Hogwarts student could end up in the Marvel universe. This creates an easy connection point for the two very different settings.
In Harry Potter, a character can be themselves by following their own moral compass rather than the strict rules of Hogwarts. For example, if they believe that house unity is more important than competition, they can act on that. In KHR, a character can be true to themselves by using their skills in a way that aligns with their personal beliefs. If they think violence is not the answer, they can use their fighting abilities to protect rather than attack.
One way to blend them is through the plot. Suppose there is a powerful dark force that is a threat to all three worlds. The heroes from Harry Potter, the Nephalem, and the characters from DxD would have to come together to fight it. The Nephalem could use their skills from their own world, like demon - slaying abilities, in the magical world of Harry Potter. In DxD, they could use the magical knowledge from Harry Potter to outwit their enemies. And the different cultures and power hierarchies from each world would create a rich tapestry for the fanfiction.
One way is through the use of magical artifacts. In a 'Harry Potter science fanfiction', a time - turner could be studied from a scientific point of view. Scientists (wizard scientists) might try to figure out the physics behind traveling through time, looking at concepts like relativity and how magic circumvents them.
Another option is FanFiction.net. Although the search there might not be as refined as on AO3, you can still find some good 'khr crossover harry potter fanfiction' by using specific keywords in the search. Also, check out some of the fanfiction communities on social media platforms like Tumblr or Reddit. People often share links to their favorite stories there.
The themes blend by pitting the light - hearted and somewhat familiar magic of Harry Potter against the mind - bending, incomprehensible horror of Cthulhu. For instance, the concept of good vs. evil in Harry Potter takes on a new dimension when faced with the amorality of Cthulhu - like beings. The wizards' sense of order and their magical laws are challenged when these chaotic and eldritch forces enter their world.
Well, in some fanfics, the honor code of the Yautja might blend with the Gryffindor's sense of bravery. A Yautja might respect a Gryffindor who shows great courage in the face of danger, just as they respect worthy opponents in their own culture.
The worlds might blend through the use of shared magical elements. For instance, both have magical creatures. The gargoyles' ability to turn to stone during the day and come alive at night could be related to some form of Transfiguration magic in the Harry Potter world.
In a fanfiction, the blending could occur in a more dangerous situation. Let's say there is a rift between the two worlds and the magic is getting mixed up in a chaotic way. The characters have to figure out how to control this new, combined magic. They might find that some spells from Harry Potter work differently when combined with the energy of the devils in Highschool DxD. For example, the Killing Curse might be affected by the demonic energy and instead of just killing, it could turn a target into a half - devil. This would force the characters to understand both magic systems deeply and find a way to separate or control the combined magic before it causes more harm.
One way is through the use of magic and advanced technology. For example, Guinan's knowledge of time - travel technology could be combined with the time - turning magic in the Harry Potter universe. This would create really interesting plotlines.