There could be a rogue musketeer who has been seduced by the allure of the dark side. He knows all the secrets and strategies of the musketeers, which makes him a very dangerous adversary as he now uses that knowledge to try and defeat them in this fanfiction.
It could be a powerful sorcerer who thrives on chaos and darkness. He uses his magic to create all sorts of problems for the musketeers, like summoning dark creatures to attack them or casting spells that make people turn against each other.
Maybe a corrupt nobleman who has made a pact with the forces of darkness in order to gain more power and wealth. He uses his position and influence to undermine the musketeers and stop them from their mission of leaving the darkness behind.
One idea could be that the musketeers discover a hidden conspiracy within their own ranks that is related to the 'leave darkness behind' concept. Maybe one of their comrades has been secretly working for an enemy and they have to figure out who it is and stop the betrayal.
The theme of transformation is very common. This can be a physical, mental or emotional transformation. A character might start off as a broken, bitter person because of their dark experiences. But as they progress through the story, they become more positive, confident and kind. This transformation is often a central part of the 'leave darkness behind' concept in fanfiction. It shows the power of change and the human spirit's ability to overcome difficulties.
Most likely, the main characters would be the Teenage Mutant Ninja Turtles themselves - Leonardo, Donatello, Michelangelo, and Raphael. They are the core of any TMNT story.
It could be Mysterio. He's known for his illusions and deception. He might create false scenarios that make the Avengers think Peter is a threat. He could manipulate the Avengers' perception of Peter's actions, like making it seem like Peter is causing destruction when he's actually trying to help.
Lex Luthor could be a likely candidate. He is extremely intelligent and has a long - standing vendetta against superheroes. He might come up with a complex plan using his vast resources and scientific knowledge. For example, he could create a weapon that targets their unique abilities. He could also manipulate other villains and heroes alike to create a situation where Batman and Wonder Woman are isolated and vulnerable.
It could be Mysterio. He's a master of illusions. He might trick the Avengers into believing that Spiderman is a danger, using his illusions to make it seem like Spiderman is doing bad things. So the Avengers, thinking they're doing the right thing, kidnap Spiderman.
In some 'leave darkness behind fanfiction', it might focus on a group of characters rather than just one. For example, a team of adventurers who have all faced their own dark moments. Together, they support each other in leaving those difficult times behind. They might embark on a quest that not only helps them individually but also strengthens their bond as a group, leading them to a brighter future.
A powerful sorcerer who wants to control the power of the fairy of darkness for his own evil purposes could be a great antagonist. He might try to trap her and force her to do his bidding.
It could be the emissary of darkness itself. The story could focus on its journey, its thoughts, and its plans.
Thanos could also be a good option. Imagine Bella's planet holds one of the Infinity Stones that Thanos needs. He would stop at nothing to get it, and Bella and the Avengers would have to join forces to protect it. Thanos' brute strength and strategic mind would pose a huge challenge for them.