A theme of self - discovery could also be present. As the character experiences being a monster at night, they might learn new things about themselves, their limits, and their capabilities. This new identity as a monster could force them to re - evaluate who they are and what they are capable of, which is a key aspect of self - discovery.
Isolation might be another theme. If the character becomes a monster, they may be isolated from normal society at night. They might not be able to interact with others in the same way as they do during the day. It could also explore the idea of the dual nature of human beings - the normal self during the day and the monstrous self at night, which shows the complexity within a person.
The concept of becoming a monster at night is really captivating. It creates a sense of mystery and dual identity. The main character has to deal with this secret side of themselves, which adds a lot of depth to the story.
Isolation and connection. The monster is usually isolated because of its nature, and the love story shows a connection being made. Also, transformation might be a theme. The love could potentially transform the monster or the person in the relationship.
One possible theme could be fear. Since there's a monster involved, fear of the unknown or the dangerous is likely.
Given the title 'rlveted in the night novel', it could also have a theme of the unknown. Nighttime often represents the unknown in literature. Characters might be dealing with things they don't understand, like strange events or new emotions that emerge during the night. There could also be a sense of danger associated with the night, so danger could be another possible theme.
One book that might be similar is 'The Monstrumologist'. It also involves elements of the strange and monstrous. Another could be 'Miss Peregrine's Home for Peculiar Children' which has a touch of the abnormal and mysterious like your mentioned novel.
One possible theme could be self - discovery. When a character goes for a night swim, they are often in a more solitary and reflective state. The stillness and mystery of the night water can make them confront their inner thoughts and feelings, leading to self - realization. Another theme might be freedom. The act of swimming at night, away from the hustle and bustle of the day, can symbolize a break from constraints and a sense of liberation.
Another theme could be the struggle between good and evil. The league representing the good side fighting against the monsters which could symbolize evil. Also, themes of sacrifice might be present as the characters in the league may have to give up things to win the war.
One possible theme could be fear. The night often represents the unknown, and in a short story set at night, fear of the dark, of what lurks in the shadows could be a major theme. Another could be isolation. At night, a character might feel more alone, cut off from the rest of the world.
The theme of self - discovery could be present. For example, a woman who takes night walks to think and figure out her life path. Also, a theme of resilience. A night woman facing difficulties at night, like a single mother working at night to support her family, and still being strong.
One possible theme could be fear of the unknown. Since the monster has no name, it represents something that is not clearly defined, which often scares people.