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.
Another option could be an ancient character from either the Harry Potter or Twilight lore. For instance, an old wizard who has been secretly observing the Twilight world for centuries and finally decides to reveal himself as the Master of Death. Or perhaps an original vampire from the Twilight world who discovers a way to gain the power of the Master of Death and uses it to gain an upper hand over other vampires and werewolves.
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.
In some fanfictions, the 'Master of Death' concept from Harry Potter might be combined with Twilight elements in really creative ways. For example, the character who becomes the Master of Death could have abilities that interact with the vampire and werewolf lore in Twilight. They might use the power over death to, say, reverse the vampiric state or bring back a werewolf from the brink of death in the Twilight - Harry Potter crossover universe.
The Master of Death could potentially use his powers to bring balance to the Twilight world. In Twilight, there is a constant struggle between vampires and werewolves, and humans are often caught in the middle. Harry, with his knowledge and power, could try to create a more harmonious coexistence. He might use his power to ensure that vampires don't overstep their boundaries when it comes to preying on humans. He could also use his understanding of life and death to help the werewolves better control their transformations, which are often tied to the cycle of the moon. And all of this would be done while exploring the new and different magic or supernatural elements in the Twilight universe.
Perhaps a dark wizard who is jealous of Harry's new power. This wizard could be someone who has long been in the shadows, waiting for the right moment to strike. He might see Harry as a threat to his own plans of taking over the wizarding world.
It could be Voldemort who somehow follows Harry into the Pokemon world. He might try to use the power of Pokemon to regain his strength and take over both the wizarding world and the Pokemon world. His dark magic could corrupt some of the Pokemon, turning them into his minions.
It could be Voldemort who somehow follows Harry into the DC universe and teams up with some of the DC villains like Lex Luthor. Their combined evil intellect and power could pose a great threat to both Harry and the DC heroes.
It could be Voldemort who somehow manages to extend his reach into the ROTG world. Since Harry is the master of death, Voldemort might see this as an opportunity to gain even more power and immortality. He could team up with some of the villains from ROTG, like Pitch, to create chaos and try to defeat Harry once and for all.
It could be a dark wizard who wants to steal Harry's power of being the Master of Death. This wizard might believe that with that power, they can rule the wizarding world without any opposition. He could be an ancient dark wizard who has been in hiding for centuries and now sees Harry as the key to ultimate power.
A new kind of magic. Harry as the Master of Death could introduce a different type of magic system compared to the existing magic in Middle - earth. His power over death might allow for some unique plot developments, like bringing back characters who have fallen in the battles of Middle - earth.
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.