Harry could use his invisibility cloak to his advantage in Thor's world. He could spy on enemies or gather information without being detected. His ability to communicate with spirits as the Master of Death might also give him an edge. He could perhaps communicate with the souls of fallen Asgardians and learn their secrets.
If Harry Potter, as the Master of Death, entered the world of Thor, he might introduce a new concept of magic. In Asgard, they have their own forms of power like the Odinforce, but Harry's magic is very different. His ability to apparate could be seen as a form of teleportation that the Asgardians might find useful. He could also use his magic to heal the wounded, which could be a great asset in the constant battles that the Asgardians face. His presence could also change the political landscape, as different factions might try to use his power for their own gain.
As the Master of Death, Harry has a certain power over life and death. In the world of Thor, this could mean he could potentially revive fallen warriors. This would be a huge deal in Asgard, where honor in battle and the loss of warriors is a significant part of their culture. Also, his magic skills could be used to create new spells or enchantments that could be useful in Thor's battles, like a spell to enhance the strength of Asgardian weapons.
Well, in such a fanfiction, perhaps Harry Potter, being the Master of Death, has some knowledge or power that Thor needs. For example, if there is a dark force in Asgard that is related to death in some way, Thor might seek Harry's help. Their encounter could lead to a cultural exchange too, with Harry learning about Norse mythology and Thor about the wizarding world.
Maybe the Deathly Hallows have some sort of connection to Asgardian artifacts. Harry, in his role as the Master of Death, starts to investigate this connection and ends up in Thor's world. Thor initially doubts Harry but as they face challenges together, like an army of dark creatures from the wizarding world invading Asgard, they become friends and allies.
Well, in a fanfiction context, one way it could be possible is through some sort of magical crossover event. Maybe the magic in the Harry Potter universe has a strange interaction with the supernatural elements in Twilight. For example, the concept of the Master of Death in Harry Potter might give Harry some sort of power or ability that allows him to traverse into the Twilight universe and interact with its characters. It could also be that a powerful wizard or vampire in a fan - created story figures out a way to merge the two concepts, perhaps using some ancient magic or vampire lore that ties the two together.
One way it could work is if the Elder Wand's power was somehow equated to a Witcher's sword. The Elder Wand is the most powerful wand in the Harry Potter world, and a Witcher's sword is crucial for combat. Harry, as the Master of Death, could use the Elder Wand's power in a new way within the Witcher's world. He could perhaps use it to enhance his physical abilities like a Witcher's mutations do. Also, the Resurrection Stone aspect of the Deathly Hallows might have an impact on the Witcher's concept of death and resurrection, which is different from the normal human view in the Harry Potter world.
One way it could work is through the concept of alternate universes. In a Marvel - Potter crossover fanfic, the power of being the Master of Death could be seen as a sort of super - power. Harry could use his mastery over death to perhaps revive fallen heroes in the Marvel universe, like bringing back Bucky Barnes before his resurrection in the main storyline. This would create a whole new dynamic and set of relationships.
Well, in a RWBY fanfiction with Harry Potter as the Master of Death, one way it could work is through a cross - dimensional portal. Harry could be accidentally transported to the RWBY universe. His status as the Master of Death might give him unique powers there. Maybe he could use the Deathly Hallows' powers to combat the Grimm. For example, the Invisibility Cloak could help him in stealth missions against the Grimm, and the Elder Wand could enhance his combat abilities against powerful foes in RWBY.
Perhaps a unique fanfiction would be one where Harry Potter, the Master of Death, has to hide his identity from the Muggle world while still helping Sherlock. He could use magic in very subtle ways that Sherlock starts to notice but can't quite figure out. This would create a lot of tension and mystery in the story, as Sherlock tries to solve the mystery of Harry's abilities while Harry is trying to keep his magic a secret.
A great crossover idea might involve Harry sharing his knowledge of death and resurrection with Thor. Thor, having faced death and loss in his own world, could be intrigued. Maybe they could work together to bring back a fallen hero from either of their worlds. Also, the Elder Wand could have some sort of interaction with Thor's hammer. What if the power of the Elder Wand could enhance the abilities of Mjolnir for a short time?
A new character created specifically for the fanfiction could also be the Master of Death. This character could be a hybrid of a wizard and a vampire or werewolf, having the combined knowledge and abilities of both worlds. They could use their power to try and create a new order where wizards, vampires, and werewolves co - exist peacefully, using their mastery over death to enforce certain rules.
Well, in a fanfiction, one way it could work is if the concept of the Master of Death from Harry Potter was translated into the world of Soul Eater. Maybe the power of being the Master of Death could give the character an edge in dealing with the Kishin and the madness in Soul Eater. For example, the invincibility that might come with mastering death could help in battles against powerful foes.