It really depends on how the fanfiction is written. If it follows the traditional power levels, Sonic with his super speed and ability to transform into Super Sonic could be very powerful. He has proven time and again to be able to take on strong foes.
Well, one possibility could be that Sonic and Eggman are forced to team up against Black Doom. Maybe Black Doom is threatening to destroy their world, and they realize that only by combining their strengths, Sonic's speed and Eggman's technology, can they stand a chance.
One great Shadow and Black Doom fanfiction could be 'The Redemption of Shadow'. It shows Shadow's struggle against the influence of Black Doom in a really engaging way. The author does a good job of exploring Shadow's character and his complex relationship with Black Doom.
In Sonic and the Black Knight story, Sonic is transported to a world based on Arthurian legend. He has to face various challenges and enemies while on a quest. He wields a sword and battles against the Black Knight and other foes to save this new world.
In the Sonic and the Black Knight story, Sonic is the obvious main character. He's the one who drives the action forward with his speed and now his sword - fighting skills. The Black Knight is a crucial character as he stands in Sonic's way and is the main source of conflict. Additionally, there could be other characters that are part of the Arthurian - inspired world. For example, there might be characters that are like the knights or ladies from the Arthurian legends. These characters may play roles in guiding Sonic or presenting new challenges for him during his journey to defeat the Black Knight.
The relationship could be one of mentorship. Lancelot, with his knightly wisdom, could mentor Sonic, who is new to the ways of the knights. Sonic would be eager to learn and Lancelot patient in teaching.
The main characters are Sonic, of course. He's the fast - running hero who gets drawn into the world of the Black Knight. And Merlina, who has her own motives and powers within this world.
In Sonic and the Black Knight story, Sonic is transported to a world of Arthurian legend. He has to face various challenges and foes while on a quest. He wields a sword and battles against the Black Knight. The story involves Sonic's speed and new combat abilities as he tries to save this strange new world.
A corrupted King Arthur could be the new villain. Imagine if the power of the Excalibur sword has turned him evil, and he now wants to rule the world with an iron fist. Sonic would have to find a way to break the curse on Arthur and stop his tyrannical rule.