There could be new original antagonists created specifically for the fanfiction. These might be dark entities that are related to the source of Dark Sonic's power, trying to either control him or use his power to spread chaos in Sonic's world.
Some fanfictions might also have Sonic's own inner demons as antagonists. Since Dark Sonic represents Sonic's darker side, the internal struggle can be personified as an antagonist. These inner demons might tempt Dark Sonic to give in to his darker nature completely and abandon his heroic values.
One possible main antagonist could be an ancient evil that has been awakened by the discovery of the Dark Excalibur. This entity might be seeking to use Sonic as a pawn to spread chaos and destruction. For example, it could be a long - forgotten dark wizard who once forged the Excalibur for evil purposes.
Well, Sonic is the central figure. But other characters like Amy Rose could be important. She might be heartbroken to see Sonic turn dark and try to use her love for him to bring him back. There could also be some new characters created specifically for this fanfiction. For example, a mysterious character who has the power to either make Sonic darker or has the key to bringing him back to normal.
There could also be some new characters introduced in this fanfiction. Maybe a mysterious figure who has the power to control Dark Sonic or someone who has a connection to the origin of Dark Sonic. These new characters can bring fresh plot twists and new relationships to the story.
In many 'dark sonic the hedgehog fanfiction' works, Knuckles is a character that shows up. He could be in conflict with Dark Sonic due to his role as a protector. And then there are other characters like Amy. She might be heartbroken to see Sonic in such a state and try to reach out to the part of him that is still good. There are also often new, original characters created by the fanfiction authors to add more depth to the story.
Eggman is by far the most common antagonist in these fanfictions. He has the motive and the means to brainwash Sonic. He's always looking for ways to defeat Sonic and gain control over the Chaos Emeralds, so using brainwashing technology on Sonic fits right into his evil plans.
One possible main antagonist could be Dr. Robotnik's sister, who is even more devious and has a different set of robotic creations. She could be jealous of Sonic's speed and popularity and wants to take over the world just like her brother, but with her own style.
In Sonic the Hedgehog fanfiction, Dark Sonic can be a really interesting addition. Fanfic writers may use him to introduce new storylines. For example, Sonic's friends could be both scared and determined to bring back the normal Sonic when he turns into Dark Sonic. Some fanfictions might also explore how Dark Sonic interacts with the villains. Maybe he's more of a threat to them than the regular Sonic, or perhaps the villains try to control him for their own evil plans.
Well, it could be about Sonic taking on a darker form or personality. Maybe there's some kind of evil influence that causes him to go dark, like a powerful dark artifact or an encounter with a really bad villain that messes with his mind and makes him turn against his friends.
One common element in the Sonic turns into Dark Sonic fanfiction could be the role of antagonists. They might have a hand in triggering this transformation. For example, by manipulating Sonic's emotions or by using some sort of dark magic. Once Sonic becomes Dark Sonic, his relationships with his friends like Tails and Knuckles also change. His friends may try to bring him back to his normal self, which can create a lot of drama and action in the fanfiction.
You could also develop his character by showing his vulnerability. Maybe there's a situation where his dark powers are not enough, and he has to face his weaknesses. This can make him more relatable and complex.