The main antagonist could be Master. Since the title mentions Master, it's likely that this character has a lot of power and influence in the merchant world. They might be using their position to cause harm or manipulate things in a way that leads to death, which would make them a natural antagonist for Harry and Tony to oppose.
It could be Death itself. Although it might seem like an abstract concept as an antagonist, in a fanfic, Death could be personified. It could be targeting the merchants for some reason, and Harry, Tony, and the Master (who might be caught in the middle as well) have to find a way to outwit Death. Death could be shown as a powerful and relentless force that stops at nothing to claim the lives of the merchants.
Obviously, Harry and Tony Merchant are the main characters. Harry could be a heroic type, with special abilities or skills. Tony Merchant might be a more complex character, perhaps a bit of a rogue but with a good heart.
One possible plot could be that Harry and Tony are both merchants in a dangerous world. Master is a powerful figure who controls the market. Death starts to target the merchants due to some ancient curse. Harry and Tony must team up, using their skills as merchants to find a way to break the curse and avoid death, while also dealing with Master's underhanded attempts to take over their businesses.
Perhaps an ancient spirit that has been awakened by the imbalance in the life - death cycle due to Harry becoming the Master of Death. This spirit sees Harry as the cause of the chaos and tries to destroy him to restore the balance in a way that is harmful to the wizarding world.
Perhaps it's a story set in a post - apocalyptic world. Harry and Tony Merchant were trying to survive. They got into a conflict with a brutal gang. During the fight, Merchant got fatally injured. Harry then had to carry on with Merchant's unfinished business and seek revenge.
I'm not sure who Tony specifically is in this fanfiction without reading it. It could be an original character created by the author, or perhaps a reimagined version of a known character in a particular fictional universe.
The 'Merchant of Death' concept probably means that Tony is involved in activities related to death in a commercial way. For example, he might be selling things that can cause death, such as weapons or poisons.
It could also be an old and powerful magical creature that has been awakened by some dark magic. This creature sees Harry as a threat to its existence because of his mastery of magic and decides to target him. For instance, a dragon that has been enchanted with dark magic to be more intelligent and malicious.
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.
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 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.