As you might have seen in my list, there are three subchapters to the main series. I thought I might explain what makes them so special but also not special enough to be a full chapter.
Very simple answer: They’re very short but they each hold some important background information.
The first subchapter, which takes place between chapter I and II, dwells around the relationship between the characters Freya and Kalem. The focus lies on the development of Freya because she plays a critical role in chapter VI, where she reappears as a lost soul. To understand her actions and her relationship to some of the chapter VI characters, you need to know how she became the way she is. She’s also part of the legendary swordmasters and that is actually already enough to make her story worth told.
The second subchapter, which also takes place between chapter I and II but many years after subchapter I, tells the story of Karsh. He simply deserved his own story arc by being one of the main villains til chapter V.
The third subchapter, however, is kinda part of chapter VI but, since chapter VI is the longest chapter already, I decided to label the backstory of the elder generation as subchapter. So it takes place before the events of chapter VI but it is told within it in the form of flashbacks.