The main antagonist could be an old enemy of the Malfoy family from the wizarding underworld. This person, filled with hatred for the Malfoys, manages to get into Hogwarts and chain Draco. Maybe it's a wizard who lost a great deal to the Malfoys in a business deal or a power struggle. Also, it could be a dark entity that has been lurking in the shadows of Hogwarts for centuries and decides to target Draco for his pure - blood magic. This entity could be using Draco's chained state to gain more power and influence within the castle.
One potential main antagonist in a chained Hogwarts Draco fanfiction could be a jealous Slytherin student. This student, envious of Draco's status and popularity within Slytherin, teams up with some dark magic users outside of Hogwarts. They manage to chain Draco in a hidden part of the castle. The jealous student believes that by getting rid of Draco, they will be able to take his place as the leader of Slytherin. However, as the story progresses, they realize that their actions have far - reaching consequences and that Draco was not as easy to replace as they thought.
It could be Voldemort or one of his Death Eaters. They might chain Draco to use him as a bargaining chip or to force his family to do their bidding. Another possibility is a powerful, dark - minded pure - blood wizard who has a vendetta against the Malfoy family and decides to chain Draco as a form of revenge. Or perhaps a rogue Hogwarts professor who has dabbled too much in the dark arts and sees Draco as a means to an end, chaining him to carry out some dark experiment.
The main antagonist could be a group of dark wizards who are trying to revive an ancient and powerful dark magic. They believe that Draco has some information about it because of his family's past associations. So they chain him up to force him to talk. This group could be led by a mysterious figure who has a personal vendetta against the Malfoy family.
Maybe it could be a new character who is a combination of ninja and wizardry evils. This character could be jealous of Naruto's unique abilities that bridge the gap between two different worlds and wants to stop Naruto from uniting the ninja and wizarding communities against him. They could have powers that are a mix of dark magic and some dark ninja jutsus.
It could be a vengeful Death Eater. Maybe Draco once crossed this Death Eater in some way, and now the Death Eater wants to teach him a lesson by locking him in a cage. This Death Eater might be jealous of Draco's status in the Malfoy family or his closeness to Voldemort at one time.
Maybe an old family acquaintance of the Blacks who has a vendetta against Andromeda for leaving the family and is now worried that Draco, as a young Slytherin from a prominent family, is being led astray by her. This character could use their knowledge of the Black family secrets and connections to create obstacles for Draco and Andromeda as they try to build a relationship.
One plot idea could be that Draco is chained in the Hogwarts dungeons due to a false accusation. Harry, against his better judgment, decides to investigate and ends up finding out the real culprit while also learning more about Draco's complex personality. Another idea is that Draco is chained as part of a dark magic experiment gone wrong, and Hermione uses her vast knowledge to try and break the chains, leading to an unlikely alliance between them. A third plot could be that Draco chains himself as a form of self - punishment for his family's actions, and a new student at Hogwarts helps him come to terms with his past and break free from his self - imposed chains.
The main antagonist could be an ancient evil spirit that has been awakened by the mixing of ninja and wizarding magic. This spirit is angry at the intrusion and starts to possess students and teachers at Hogwarts. The ninja and Harry Potter must figure out how to exorcise the spirit before it's too late and Hogwarts is completely taken over.
It could be a new dark wizard who is jealous of Harry's new mastery. This wizard might try to steal Harry's knowledge or use it for his own evil purposes. He could be a former Death Eater who was thought to be dead but has now returned with a vengeance.
It could be a new dark wizard who has been lurking in the shadows. Maybe someone who was once a student at Hogwarts but turned to the dark side.
It could be a dark wizard who has infiltrated Hogwarts' staff. This wizard might be secretly manipulating events and students for his own dark purposes. He could be using his position to spread dark magic teachings and trying to turn students against each other, all while Lily Luna Potter starts to suspect something is wrong and tries to expose him.
The main antagonist could be an old, corrupt house - elf. This house - elf was mistreated in the past at Hogwarts and has now turned to the dark side. It wants to bring chaos to Hogwarts and sees Lady Hogwarts as the key. It uses its knowledge of the school's hidden passages and secrets to plot against her. Harry Potter, being the defender of the underdog, tries to reason with the house - elf at first but when that fails, he has to use magic to stop it from harming Lady Hogwarts.