Authors often start by finding a connection point. For example, a magical portal could be the link between the Harry Potter world and Sesshomaru's world. This allows for the characters to meet in a somewhat believable way.
Writers often start by establishing how a fem Percy Jackson could end up in the Harry Potter universe. Maybe through a magical portal or some kind of cross - dimensional spell. Then they find ways to make her demigod powers interact with magic. For example, her ability to control water might have an unexpected effect on a magical water - based spell in Hogwarts.
In some fanfictions, they merge through magical artifacts. For instance, the Sword in the Stone could be related to the Sword of Gryffindor. Maybe the two swords have a shared origin or power, and this connection brings the two worlds together. Characters from Harry Potter might be drawn to Camelot to discover more about this connection.
Typically, they use some sort of magical or dimensional rift to merge the two worlds. For example, Naruto could be sucked into a portal that leads him to the High School DxD universe. Or, it could be a more complex plot device like a powerful villain trying to combine the two worlds for their own gain.
In many fanfictions, magic is the key to merging the two universes. Fem Harry Potter's world of magic can act as a bridge. For example, a powerful spell might accidentally open a portal between the wizarding world and Metropolis, allowing the two to interact.
One way the worlds could merge is through a magical portal. Maybe a Hogwarts student accidentally discovers a portal that leads to the world of Princess Mononoke. The student could then introduce magic from the Harry Potter world to the people in Mononoke's world, like using spells to heal the forest spirits that are sick or injured. Another aspect could be the sharing of beliefs. The respect for nature in Princess Mononoke's world could influence the wizards in Harry Potter to be more in tune with the natural magic around them.
The worlds could merge through a magical portal. For instance, a wizard in the Harry Potter world accidentally creates a portal to the land of Jack the Giant Slayer while experimenting with a new spell. Then, characters from both sides start to interact. The wizards could teach the people in Jack's world about magic, and Jack could teach them about his skills in fighting giants.
In these fanfictions, the merge often starts with a crossover event. For example, a portal might open between Danny's world and the world of Tangled. This allows the characters to interact. They might use their unique abilities to help each other. Danny's ghost powers can be a novelty in Tangled's world, and Rapunzel's long hair and her knowledge of her own world can assist Danny in his adventures in her world.
Authors may create unique worlds in 'Harry x Merlin' fanfiction by merging the cultures of the two series. They could combine the wizarding society of 'Harry Potter' with the medieval - like court of Camelot. This would mean new social hierarchies, traditions, and ways of life for the characters. For example, Harry might have to learn the proper etiquette of Camelot's court while Merlin has to adapt to the more modern - for them - ways of Hogwarts. This cultural mash - up can lead to very engaging and distinct fanfiction worlds.
Some fanfictions merge the universes by having a shared event. Like in 'The Great Crossover Event', there is an event that draws characters from both the Harry Potter universe and the Yu - Gi - Oh universe together. Fem Harry Potter uses her magical skills, and she has to learn about the dueling system in Yu - Gi - Oh to participate. It's all about how these two different types of magic and combat styles interact with each other.
The blend often occurs at a character level too. Characters from both series might form alliances or rivalries. A friendship between Legolas and Hermione could develop, with each sharing their knowledge of their respective worlds. Or perhaps Snape and Saruman could be pitted against each other, their cunning and knowledge of dark magic making for a great showdown. These character interactions help to bring the two very different worlds of 'Lotr' and 'Harry Potter' together in the fanfiction.