Since I don't know the exact 'Dark Knight Castle Story', but it could have the dark knight as a main character, of course. Maybe there's an old wizard who lives near the castle and is either an ally or an adversary to the knight.
I'm not sure specifically which 'Dark Knight Castle Story' you're referring to. There could be many stories with such a name. It might be about a knight in a dark and mysterious castle, facing various challenges like fighting monsters or solving mysteries within the castle walls.
He could react with anger. After all, if he has feelings for Beckett, seeing her with someone else like Vaughn might make him furious. He might lash out at both Beckett and Vaughn without really understanding the situation.
Castle would probably be shocked at first. He'd be thinking 'Why is my daughter doing this?'. Then he'd start trying to talk to her, using his charm and wit to figure out what's going on.
You can start by searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. Just type in 'Castle' in the search bar and then browse through the results to find ones that seem relevant to your viewing experience.