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.
It could be a new, powerful and malevolent god. This god might be jealous of Percy's status as a hero and the favor he has from some of the Olympian gods. So, the new god devises a plan that makes Percy's life so miserable that he asks for death.
Annabeth could be one of his main allies. She is smart and would help Percy understand the more complex aspects of his role as Horseman of Death. Grover might also be there for him, using his nature - related powers to help Percy in places where the power of death interacts with the natural world.
In Percy Jackson fanfiction, the 'Master of Death' could refer to a character who has control over death in some way. Maybe they can cheat death, like Voldemort in Harry Potter who was after the Elder Wand to become the Master of Death. In the Percy Jackson universe, it could be a demigod who has some special power over the Underworld, perhaps being able to bring back the dead or prevent deaths that are fated. It's a really interesting concept that fanfiction writers can play around with to create unique and exciting stories.
A primordial being related to fire, like a forgotten Titan of fire, could emerge from the depths of Tartarus. It could be angry that the Olympians have been ruling for so long and sees Percy's new fire powers as a threat or a tool to use against them.
Percy himself could be a potential sword master in fanfictions. Given his skills with Riptide, he could further develop his swordsmanship under a new mentor. Another could be Annabeth. Her strategic mind would make her a great sword master as she could combine her intelligence with combat skills. Nico di Angelo is also a possibility. His dark powers could be incorporated into his sword - fighting style, making him a very unique and powerful sword master in fanfiction stories.
Well, in some Percy Jackson fanfiction, there might be other characters who are made to be the master of water. Maybe a new character is introduced who has some unique ability related to water that surpasses Percy's. However, usually, Percy is the main character associated with water powers because of his heritage. He can summon water, control its currents, and even communicate with water - based creatures. So, in most cases, when we think of the master of water in Percy Jackson fanfiction, Percy comes to mind first.
It could be an ancient and powerful Titan who escaped from Tartarus when the Doors of Death started to malfunction. This Titan wants to keep the doors open to bring back all the old enemies of the gods and take over the world again.
Well, sometimes in fanfiction, his brother might be a mortal who discovers his demigod heritage later. This could add an interesting twist to the story as he has to adapt to the world of gods and monsters. Or it could be a long - lost half - brother from a different camp, like a son of Poseidon from another part of the world who was hidden away for some reason.
One possible plot could be Percy being chosen as the Horseman of Death due to some ancient prophecy. He might have to travel to the Underworld more often and deal with the spirits there in a new way. Maybe he has to collect the souls of those who are supposed to die but are being held back by dark magic.
She might be a character created to add a new twist to the story. For example, she could be a demigod with a very rare power that no one has seen before. Ariana could be a key to solving a big mystery in the fanfiction world. She could also be an antagonist at first, but then later become an ally to Percy after some character development.