Perhaps it's a new dark creature that has emerged in the Black Lake. This creature sees Harry as an easy target and takes him hostage, either for food or for some other unknown reason. It could be that this creature has some sort of grudge against humans or wizards in particular.
Well, one possible plot twist could be that Harry discovers he has a hidden power that allows him to communicate with the creatures in the Black Lake. This power could help him either escape or turn the situation around against his captors.
I'm not sure specifically as it could be a very unique fan - made story. It might involve a character named Draco being held hostage near a lake in the context of a fictional world created by the fanfic writer.
I'm not sure specifically as I haven't read this particular fan fiction. But generally, it might involve a rescue mission to save George from the hostage situation in or near the lake, with some romantic elements intertwined, like a character having romantic feelings for George and being the one to bravely attempt the rescue.
Well, first, check the major fan - fiction platforms. Archive of Our Own is known for having a vast collection of fan fictions. Another option is Wattpad, although it has a broader range of content. Search for the title 'draco is a lake hostage fan fiction' on these sites. Also, some fandom - specific forums might have links or discussions about this particular fanfic, so it's worth looking into those as well.
Since I haven't read it, it's hard to say for sure. But it could be a friend or a love interest. Maybe a person named Mary who has a crush on George and will do anything to save him from the hostage situation at the lake.
One motivation could be simple friendship. Despite being from different schools, they might have developed a bond during the Tri - Wizard Tournament. Viktor could see Harry as a worthy opponent and not want to see him die in the second task.
In the fan fiction, Viktor might use his excellent swimming skills. He is from Durmstrang after all, which is by the Black Lake. He could have swum quickly to Harry when he was in trouble in the water, grabbed him, and swam back to safety.
One common theme could be rescue. Since Draco is a hostage, there might be a focus on who will rescue him and how. Another theme could be his inner struggle while being held hostage. He might be dealing with fear, helplessness, or trying to find a way to escape on his own.