Most likely, the 'terror' itself is the antagonist. It's what the hero has to fight against.
It could be a character or entity that embodies the terror. For example, a powerful and malevolent sorcerer who spreads fear and chaos, and that would be the main force that the hero has to oppose in the 'hero and the terror novel'.
There's a possibility that the hero is a misfit or an outcast in society. This person has always been different and not fully accepted, but they possess unique qualities that make them the perfect candidate to be the hero the world needs. They might start off being underestimated, but as the story unfolds, they prove themselves to be the one who can change the world.
The antagonist could be a super - villain who has a personal vendetta against the hero. He might have used his influence to make the hero's life a living hell and made everyone think the hero was worthless, like garbage. The hero's revenge is all about taking down this super - villain and restoring his own honor and the safety of the world that the villain has been threatening.
An organization that opposes the hero's unorthodox methods could serve as the antagonist. They might believe in a more traditional way of heroism and see the hero as a threat to their established order. They could try to stop the hero from using their unique abilities or ways of doing things.
It could be a rogue ninja who was once a student of the Hiraishin master but turned evil. They might have a twisted view of the technique and use it for their own selfish and harmful purposes, like stealing important artifacts from different villages.
One potential antagonist could be a rogue ninja who also specializes in bojutsu. He might be jealous of the main character's skills and tries to undermine him at every turn. Another option is a powerful villain from the Naruto universe who sees the bojutsu master as a threat to his plans.
A Sith apprentice could be a great antagonist. They might be after the same artifacts or information that the Padawan and Ahsoka are seeking. For example, if there's a powerful Sith holocron, the Sith apprentice would stop at nothing to get it.
Stain could be considered as an anti - hero in fanfiction. His extreme views on heroes, while not entirely acceptable, could be delved into deeper. He believes that only true heroes should be recognized and is willing to take extreme measures to prove his point. A fanfiction could explore a different side of him, perhaps one where his actions are more nuanced.
It could be a modern - day villain who wants to use the power of the ancient heroes for his own evil purposes. He might be jealous of Percy's connection to the ancient heroes and try to stop him at every turn.
It could be another god - like being who is jealous of the female character's powers or her influence in the Naruto universe. This antagonist could be from the same realm as the female character but has different motives. He or she might want to use the chaos in the Naruto world to gain more power for themselves, and the female character stands in their way.
Perhaps an ancient evil that Gryffindor once sealed away is starting to break free. This entity sees the descendant as a threat because they have the potential to reseal it. So, it tries to corrupt the descendant or stop them in any way possible.