A common theme is the battle between good and evil. In 'Harry Potter', Harry battles Voldemort; in 'Buffy', she fights vampires and other demons; and 'Morningstar' also has elements of this struggle. So in fanfiction, authors often create scenarios where characters from these different universes team up to fight a greater evil.
Love and relationships are also quite popular. Writers might pair characters from different universes in romantic relationships. For example, a character from 'Harry Potter' might fall in love with a character from 'Buffy'. This can bring a whole new dynamic to the story as they come from different worlds with different values and cultures. It also adds an element of intrigue as they try to navigate their relationship while also dealing with the threats and challenges from their respective universes.
Well, Archive of Our Own (AO3) is another excellent source for fanfiction. It has a very comprehensive tagging system. You can search for 'Harry Potter' and then look for works that might also mention 'Morningstar' or 'Buffy' in the tags or descriptions. Some authors like to create crossovers between different fandoms, and AO3 is a great place to discover these unique and often very creative fan - made stories.
One common theme is the battle between good and evil. Both Harry Potter and Buffy are all about fighting against dark forces. In fanfiction, this often means combining the Death Eaters or Voldemort from Harry Potter with the vampires, demons and other baddies from Buffy's world.
Power and responsibility is also a big theme. Both Harry and Buffy have a great deal of power, Harry as a wizard and Buffy as a slayer. In the fanfiction, they often have to grapple with how to use their power in the right way, especially when they are in an unfamiliar situation like when Buffy is in the wizarding world or Harry is in Sunnydale.
One common theme is the clash of different magical systems. Harry Potter's magic is more structured and based on spells and wands, while Buffy deals with more instinctive and primal supernatural powers. When these two worlds collide in fanfiction, exploring how they interact and conflict is a big theme.
Harry Potter himself is often a popular character in these crossovers. His connection to magic and his hero status in his own world make him an interesting figure to pair with Buffy. He can bring his knowledge of spells and the wizarding world to the mix.
One common theme is the exploration of power. Since the 'Master of Death' concept involves a great deal of power, it's often about how the characters handle it. Another theme is crossover elements, like mixing the magic from 'Harry Potter' with the supernatural fighting in 'Buffy'.
Harry's Patronus charm, which is a manifestation of positive energy, could have an interesting reaction when in the presence of Lucifer. Since Lucifer is associated with the underworld and negative forces in a way, the Patronus might act as a counterbalance or even a repellent to some of the darker aspects of Lucifer's world. At the same time, Lucifer's power to see people's desires could help Harry understand the motives of his enemies better. For example, if they were facing a dark wizard who had some hidden desires that were fueling his actions, Lucifer could reveal those, and Harry could then use that knowledge to find a way to defeat the wizard using his magic.
One interesting crossover could be a scenario where Harry Potter is transported to Lucifer's world in Los Angeles. Maybe he needs Lucifer's help to defeat a new kind of dark magic that has some connection to the infernal realm. For example, a dark wizard has made a pact with a minor demon and is causing chaos in both the wizarding world and the human world. Harry, with his knowledge of magic, and Lucifer, with his understanding of the supernatural in his own realm, could team up to solve the problem.
They can be combined by having a common enemy. Suppose there is a new dark force that threatens both the wizarding world and Lucifer's world. Harry Potter and Lucifer Morningstar would then have to team up to fight it. This would bring together their different powers and ways of thinking. Also, a character from one world could be cursed to live in the other world, which would create interesting storylines as they try to adapt and find a way back.
Well, in some fanfictions, Lucifer Morningstar could be curious about the magic in the Harry Potter universe. He might visit Hogwarts in disguise and cause chaos. He could interact with Dumbledore, who would be very cautious of him. There could also be a fanfiction where Harry Potter and his friends have to go to Hell (Lucifer's domain) to retrieve a lost magical artifact and face all kinds of devilish challenges along the way.