One possible theme could be family. Since he is the son of a marquis house, family dynamics like loyalty, inheritance, and family pride would likely play a role. Another theme might be power, as being part of a marquis house involves political power and influence.
There could be themes of self - discovery. The son, in the course of the novel, may be trying to find out who he really is and what his place is in the world. Also, themes of honor and duty are very likely. As a member of the marquis house, he has certain responsibilities towards his family and the society he lives in. This could lead to internal conflicts within him as he tries to balance his own desires with his duties.
Well, in the 'legendary youngest son of the marquis house novel', it could center around the relationships the youngest son has. He may have a complex relationship with his parents, who might have different expectations of him compared to his older siblings. His relationships with the servants in the marquis house could also play a role. He could befriend some of them and they might help him in his endeavors. Also, there could be a romantic sub - plot, where he falls in love with someone from a different social class or a rival family, which would add more drama to the story.
In addition to the family members, there could be a mentor figure. This could be an old family retainer who has seen many generations in the marquis house and takes the youngest son under his wing. There might also be antagonists. For example, a jealous relative from another branch of the family who wants to undermine the marquis house and sees the youngest son as a key to achieving that. And if there's a political or social context, there could be nobles from other houses who interact with the main character in various ways.
I haven't read this novel specifically. But generally, a novel with such a title might be about a male protagonist from a marquis house who has some extraordinary qualities or experiences that make him 'legendary'. Maybe he has to face political intrigues, family disputes, and at the same time, he may have some great achievements in martial arts, magic or other aspects in the story.
One possible theme could be mortality. Given the title 'the house of death', it likely explores the concept of death and the end of life. Another theme might be fear. A house associated with death would surely evoke fear in the characters and readers alike.
Family could be a major theme. Since it's about a son, his relationship with his parents, siblings might be explored. Another theme could be morality. As he is 'the good son', what does it mean to be good in the context of the story? Growth might also be a theme, like his personal growth as he faces different situations.
Nature could be a theme. The marsh environment around the house can play a big role. It might be a source of beauty, danger, or a symbol of the untamed. For example, the marsh could represent the chaos that the characters' lives are in or it could be a haven from the outside world. Also, family might be a theme. If there are characters living in the house, their family relationships, secrets, and dynamics could be explored in the novel.
One possible theme could be the concept of a nurturing environment for mothers. It might explore how a physical house can be a place of growth, love, and support for mothers.
One possible theme could be identity. Since it's an Austrian novel, perhaps the characters in 'The Red House' are grappling with their Austrian identity in a changing world.
One possible theme could be family. The house on Malcolm Street might be the backdrop for a family's story, showing their bonds, conflicts, and how they interact within the walls of that house.
One possible theme could be family bonds. Since it's set in grandma's house, it might explore the relationship between different generations in a family.