Who could be the main antagonist if Naruto is sent to hell in fanfiction?

It could be the Devil or some powerful demon lord in hell. This antagonist would try to keep Naruto trapped in hell and prevent him from escaping. He might see Naruto as a threat to his rule in hell or just enjoy tormenting him.

It could be an escaped soul from the King of Hell's domain. This soul might have a grudge against Naruto or the ninja world in general. Maybe it was wrongly imprisoned and now wants revenge by causing chaos in the living world.

It could be an ancient demon who has ruled hell for centuries. This demon might see Naruto's attempts to bring change and order to hell as a threat to his power. So, he tries to stop Naruto at every turn, using all the dark powers at his disposal like summoning hordes of hellish creatures or casting powerful curses.

There could be a group of hellish bureaucrats who are the antagonists. They are the ones who sent Chloe to hell in the first place and are trying to keep her there to maintain the order of hell as they see it. They might not want Chloe to expose the unjust system that exists in hell.

It could be the warden of the Blood Prison. He might be a corrupt figure who is using the prison for his own gain. He sees Naruto as a threat to his operations and tries to keep him locked up at all costs.

It could be someone who misuses the power of the Hell Girl. Maybe a rogue ninja who discovers the way to summon the Hell Girl and uses her to get rid of his enemies in the ninja world. He doesn't care about the consequences or the morality of it, and Naruto has to stop him from causing more chaos.

The main antagonist could be a rogue ninja who has infiltrated the village's council. This ninja could be secretly manipulating things behind the scenes to get Naruto sent to prison. He might have a vendetta against Naruto for some past encounter or just see him as a threat to his own evil plans.

Maybe a disgruntled former member of the Glee club who feels wronged. This person has a vendetta and uses their knowledge of the club's weaknesses. They lurk on the 'highway to hell' and try to sabotage the club's journey, for example by stealing their sheet music or spreading rumors among the members to cause internal strife.

In a 'Naruto sent to hell' fanfiction, it could be that Naruto is sent to hell as part of a test. The gods or some powerful entities want to see if he can bring hope and change to the damned souls in hell. So, Naruto starts to teach the souls in hell about friendship, perseverance and the ninja way. He tries to reform the place from within, facing many challenges along the way like powerful hell guards and the dark energy that tries to corrupt him.

It could be an ancient and powerful Titan who was thought to be defeated long ago. Since Percy is in the past, this Titan could be in his prime and see Percy as a threat to his plans of ruling over the world again. For example, Kronos might be even more powerful in this past era and Percy has to find a way to stop him without the help of his modern - day friends.