There isn't enough context to accurately tell this story. However, if we assume 'black cock' is a character in a made - up tale, perhaps he lost his girlfriend because of his own bad behavior. For example, if he was too possessive or didn't pay enough attention to her needs. This could lead to arguments and eventually her leaving him.
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.
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.
Perhaps a powerful dark mage who has been observing Asta. When he loses control, this mage could try to manipulate Asta's out - of - control anti - magic to his own advantage. He might want to use it to destroy the Magic Knights or gain power over the entire kingdom. This mage could be someone who has a vendetta against the Clover Kingdom and sees Asta as a useful tool in his evil plan.
First, you need to know the characters well. Read up on all the official stories about Shadow and Black Doom. This will give you a good base to start from. For example, understand Shadow's origin and his powers, and also how Black Doom functions as an antagonist. Then, come up with a unique plot idea. Maybe something like Shadow discovers a hidden weakness of Black Doom.