In many of these fanfictions, the demons are the main antagonists. They are the ones who carry out the torture on Sam. They might have different motives, like revenge for Sam's past actions against them or simply following Lucifer's orders.
Some fanfictions introduce original characters as antagonists. These could be powerful demons or other hellish creatures that have their own reasons for wanting to torture Sam. Maybe they see Sam as a threat to their power in hell or as a means to gain more power by breaking him.
Often, Lucifer himself can be a major antagonist. He has a personal vendetta against the Winchesters, and in the context of Sam being tortured in hell, he could be the mastermind behind it all. He might be using the torture to break Sam's will and turn him into a weapon.
Some fanfictions focus on the nature of the torture itself. It could be physical, like being burned repeatedly, or psychological, such as reliving his worst memories over and over. The theme of Sam's transformation during this torture is also common. He might become tougher, more cynical, or even find a new sense of purpose in the face of such extreme pain.
It could be the traditional villains from the original story. Maybe some of the old enemies of Percy who have a vendetta against him. For example, the Titans or some rogue demigods who oppose him for power or personal reasons.
Another option is a group of demons who have a vendetta against Clary or the shadowhunters in general. They might use torture as a way to break her spirit and gain information about the shadowhunters' defenses.
It could be the traditional villains from the original story, like the Titans or some powerful monsters. They might torture Percy to get information or just to cause chaos.
Their friends could also be antagonists in a sense. For example, if there's a friend who has always had a crush on Freddie and is jealous of Sam. This friend could spread rumors about Sam being a bad influence or try to convince Freddie that he's making a huge mistake by staying with Sam and having a baby. Another scenario could be that their friends don't take the situation seriously and keep pressuring them to act like they used to, which makes it hard for Freddie and Sam to focus on the pregnancy.
There could be a powerful overlord in Hell who rules the place. Naruto's presence and his potential to grow strong might be seen as a threat to this overlord's power. So, this overlord would be one of the main antagonists, constantly sending minions or directly confronting Naruto to keep him in check.
It could be that the very concept of the hellish order itself is the antagonist. If Naruto wants to bring a more positive change, the established rules and the entities that enforce them would be against him. For instance, the guardians of the different levels of hell might not want Naruto to interfere with their way of doing things and would fight to keep the status quo.
Well, in a lot of fanfictions, demons are likely kidnappers. They might want to use Sam for some dark ritual or to get information about the Winchesters' hunting secrets. Demons have always been a big threat to the Winchesters, so it makes sense for them to be the kidnappers.
Another possibility is that there is a mysterious figure who has some sort of vendetta against the family. This figure could be causing strange and unexplained events to occur around the baby, like making the baby sick in a way that doctors can't figure out at first. This would then be a source of conflict as Bella and Sam try to protect their baby from this unknown threat.
Some of the more powerful demons in Hell itself could oppose Percy. They might be jealous of his sudden rise to power as the Prince of Hell. These demons could have their own agendas and territories in Hell that they don't want Percy interfering with. They could use their dark magic and legions of lesser demons to try and get rid of him.