A third possible theme is the power dynamic. If the 'Black Widow' has some sort of power over others, whether it's through charm, money, or other means, the story could explore how she uses that power. It might show how the power imbalance affects the relationships she has and the actions she takes in the story.
I'm not very familiar with the specific 'Black Widow of La Porte Story'. It might be a local or relatively unknown story. Maybe it involves a female figure in La Porte who has some sort of dark or mysterious reputation, like a femme fatale type, but without more information, it's hard to say more.
The scariest element might be the unknown. In the 7th sea, there are so many uncharted areas, and in these stories, those areas could hold all kinds of terrors like unseen monsters or dark magic.
Well, one horror story could be about a haunted ship in the 7th sea. The ship was said to be cursed, and any sailor who boarded it would hear strange whispers and see shadowy figures. Eventually, they would go mad.
Perhaps there's been some sort of miscommunication or misunderstanding among Peter and his friends. Maybe a villain has manipulated the situation in such a way that his friends now think Peter has betrayed them, so they've cut off all ties with him. This would create a very lonely and difficult situation for Peter in the fanfiction.
Peter could be just an ordinary person with an extraordinary love for dogs. He might be a local boy who has always been fascinated by the tower and decides to bring his furry friends along on his adventure.
Peter could be a disgruntled noble in the fanfiction. He might be someone who has been overlooked for promotions or honors by the emperor, and this has led to his resentment and desire to kill the emperor.
Again, without more context, it's hard to say exactly who Peter Braestrup is in this 'big story'. He could be an author, an activist, or someone who made a mark in a particular industry.