Maybe Darkseid. Since he is a powerful being in the DC universe and is often associated with concepts of death and destruction on a large scale. With Harry being the master of death, Darkseid might see him as a threat to his plans of universal domination and thus target him, making him the main antagonist.
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.
The main antagonist could be a new character created specifically for this fanfiction. A being that has the power to manipulate the boundaries between life and death in both the Harry Potter and DC universes. This character could be jealous of Harry's status as the master of death and wants to take that power for itself, causing chaos and destruction in both worlds as it tries to achieve its goal.
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 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.
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 an old follower of Voldemort who was in hiding. This person finds out about the Eyes of Death and believes that with their power, he can bring back Voldemort's legacy even stronger. He becomes the main force against Harry and his friends throughout the fanfiction.
It could be Voldemort again, but in a more powerful and death - centered form. Maybe he has found a way to fully embrace the power of death and become an even greater threat. Since Harry is the champion of death, it would be a battle of two opposing forces related to death.
One possible plot for this fanfiction could be that the DC universe is facing an otherworldly threat that has something to do with death and the afterlife. Harry Potter, as the master of death, is drawn into this situation. He has to navigate the complex politics and powers in the DC world while also dealing with his own powers related to death. He might meet characters like Batman, who is initially skeptical of his magic but then comes to respect his abilities as they work together to save both worlds.
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 Voldemort still. Even if Harry becomes associated with death, Voldemort is the ultimate symbol of death and destruction in the wizarding world. Their battle could take on a new dimension as Harry now has a different relationship with death.