I'm not sure as I haven't read the novel specifically. It could be someone with a complex and perhaps dark personality.
Perhaps it could be a corrupted version of one of the existing characters. For example, a character who has been influenced by dark magic and has turned into this 'prince of darkness'. It could be a Slytherin student who has delved too deep into the forbidden knowledge of the house and has become a dark force. Another possibility is that it's a long - lost relative of a major villain like Voldemort, carrying on his legacy in a new and terrifying way.
It could be an ancient and powerful dark wizard who has been dormant for a long time. Maybe someone from the time before Hogwarts was even founded.
I don't know who the author of this novel is. You could try searching for it in a library database or on a book - selling website.
What kind of novel do you want to answer? Is it fantasy, romance, science fiction, history, fantasy, or some other type of novel?
Some of the teachers at U.A. might also be antagonists. They could be secretly working for an organization that wants to control Izuku's powers for their own evil purposes. They would pretend to be helpful while actually setting traps and spreading misinformation about him.
The main character is likely the Dweller in Darkness itself, as the story is named after it.
The main character in the 'invader of darkness story' could be a person who has been wronged by the darkness in some way. Maybe they lost their family or home to the encroaching darkness. This would give them the motivation to invade the darkness and seek revenge or restoration. It could also be a character who is simply curious about the unknown and the power that the darkness might hold, so they embark on this dangerous journey as the main protagonist of the story.
Many of the powerful noble houses would be his enemies. For example, the Lannisters, who are already in a struggle for power. They would not want to share or lose their power to the Prince of Darkness. And the Tyrells, who are also politically astute and would view the Prince as a threat to their influence and the stability of the Seven Kingdoms.
The obvious choice would be the Prince of Darkness himself. He could be a powerful being with the ability to manipulate shadows and dark magic, constantly trying to draw Percy to the dark side.
Another antagonist could be a human with a grudge against Danny. This person discovers Danny's 'Prince of Darkness' potential and decides to use it against him. They might team up with some lesser ghosts and use dark magic or technology to try and take Danny down. They could be motivated by jealousy or a desire for power.