There might be a group of bandits or thieves who have made the tunnels their lair. They don't want the priestess to interfere with their activities or discover their secrets. They would use their knowledge of the tunnels to ambush and attack her, and they could be quite a formidable foe as they are familiar with the terrain.
It could be a powerful dark mage who controls the tunnels and doesn't want the priestess to succeed in her mission.
Maybe there is an ancient evil entity that has been lurking in the tunnels for ages. It sees the young priestess as a threat to its existence or its plans. This entity might have the power to manipulate the environment of the tunnels, creating all sorts of obstacles for the priestess. It could also try to corrupt her mind and turn her against her own beliefs.
Maybe it's about her facing various challenges in the tunnels, like encountering strange creatures or uncovering long - lost secrets.
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 a jealous rival. Maybe someone who has always been in competition with the young miss and doesn't want her to gain more power or attention once her secret identity is revealed.
I don't know since I haven't read the book. Maybe there's a young adventurer who discovers the tunnels of time.
I don't know as I haven't read it. It could be a group of adventurers or some kids who get lost in the tunnels.
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.
One possible main antagonist could be the ruler of hell. This powerful being would see Naruto as an intruder and a threat to the order of hell. Another could be a demon who has a vendetta against Naruto for some reason, perhaps because Naruto's positive energy is anathema to the demon's evil nature.
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.
Perhaps one of the Titans who were banished to Tartarus. They might see Percy as a threat to their plans of revenge or escape. Kronos, if he still has some form of existence in Tartarus, could be a major antagonist. He would have his own motives and powers, and he could try to manipulate Percy or simply get rid of him.
Perhaps Kaguya Otsutsuki is resurrected again, but this time with a stronger will and more dangerous plans. Since Naruto was the one who played a major role in defeating her before, she targets him first in this fanfiction.