It could be an ancient shinobi who has mastered some kind of death - related jutsu and has ascended to a higher state of being. This shinobi might have been forgotten in the history of the ninja world.
Perhaps it's a spirit that has been watching over the ninja world for ages. It could be related to the Sage of Six Paths in some way. Maybe it was created when the concept of death was first established in the ninja world. It could have its own motives for meeting Naruto, like seeing Naruto as the key to restoring balance between life and death in the world.
Well, Naruto might gain some extraordinary powers from the master of death. Maybe he could learn new jutsus or get some insights about the afterlife.
One possibility could be Doctor Strange. With his vast knowledge of the mystic arts, he might stumble upon the power of the 'Master of Death'. His already existing powers over magic and the multiverse could be a stepping stone to this even greater power.
Well, it could be about Naruto obtaining the power of a master over death. Maybe he can bring back the dead or has the ability to control deathly forces. He might use this new power to protect his village from greater threats that normal ninja skills can't handle.
In this fanfiction, Naruto is the central figure. However, other important characters could include Kakashi. Kakashi, being Naruto's former teacher, would be interested in Naruto's new power and would try to guide him on how to use it responsibly. There might also be new characters introduced, like some sort of death spirit or deity that has a connection to Naruto's power. These new characters could play a role in either helping or hindering Naruto as he tries to master his role as the master of death.
Nico di Angelo has a strong connection to the Underworld, so he's a prime candidate. He can already communicate with the dead and has some control over the shadows, which are related to death in a way. In fanfiction, he could be further developed to become the Master of Death, perhaps by making a deal with Hades or finding a hidden power within himself.
Perhaps it could be a character like Bran Stark. He has the power of the Three - Eyed Raven, which gives him access to a vast amount of knowledge and some control over time and events. It's not a far stretch to think that he could develop into a Master of Death in a fanfiction.
The idea of the Master of Death in a Harry Potter - Naruto fanfiction might involve a crossover of abilities. For example, a character like Harry Potter who has mastered death could potentially teach the ninja in Naruto's world about different ways of facing mortality. It could also lead to some unique battles where the rules of both universes are combined. Like, using magic against chakra - based attacks while having the immunity to death that comes with being the Master of Death.
Voldemort could also be a main character in a different way. Instead of just being the villain, he could be a complex figure who is also in the 'game' for the master of death's power, but with his own motives and backstory that are explored more deeply in the fanfiction.
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.
Bellatrix Lestrange is also a likely candidate. Her encounter with Hermione would be extremely intense. Given her loyalty to Voldemort and her rather unhinged nature, she would likely pose a serious threat to Hermione, but Hermione might also find a way to use Bellatrix's madness against her.