Well, it could be about Harry having a life debt towards Daphne. Maybe he saved her life in some dangerous situation, like a battle against dark wizards. And then, as a result of this life debt, they are bound in a special way. Daphne might be a Slytherin who has to deal with her own prejudices about Gryffindors, especially Harry. Their relationship could develop as they try to figure out how to handle this life - debt situation, perhaps leading to an unexpected friendship or more.
The 'harry and daphne life debt fanfiction' might start with the event that causes the life debt. It could be that Daphne was in a life - threatening situation in the Forbidden Forest and Harry came to her rescue. After that, the story might focus on how Daphne tries to repay Harry in her own way, which might not be a straightforward process. There could be interference from other characters, like her Slytherin friends who don't trust Harry. As they navigate through this complex relationship, they might discover new things about each other and the wizarding world around them, and the story could be filled with adventures, secrets, and a growing bond between them.