The Soul of Chaos is an original high fantasy with touches of adventure that will likely appeal to many readers. It has a unique world, great pacing, and some twists to keep you engaged all throughout. I'll try to keep this review non-spoiler but informative, so I may remain vague at some points.
First of all, I found the narration very well done to keep both the pace moving, and build up the world.. You are following two main characters that narrate in limited third person, and there are the occasional POV jumps to two-three more secondary characters. The chapters and the POVs are not named (e.g., the chapter is not called with the narrating character's name) but they are easy to follow--either the characters are in different places or it starts following them, which makes the switching POVs straightforward.
After that, we have epistolary mini "chapters" of 1-2 paragraphs each. These epistolary sections are either excerpts from in-book books (litanies), diaries, and so on, that build up the world--for example, an excerpt of someone making a discovery, or a letter of a mother to a child. They are all relevant, and help you understand the magnitude of some events without being told. It is also quite useful because the reader is not bombarded with lore, yet these mini-chapters are curious enough to add worldbuilding, demonstrating the depth of the world.
That is my second plus point--the worldbuilding. This book has that feeling of TTRPG and RPG since you have greatswords, combat healers, warcasters, paladins and heavy-armoured knights, and classes and professions... yet everything is so neatly tied together. There are so many names, yet the author introduces them perfectly. Somehow, you can infer what this in-book word means by the context/paragraph in which it is introduced, and then as the story progresses, you find bite-sized bits of information that continue to add to the worldbuilding.
I found this engaging, because the world is *huge* (and you can see it also has a clear past, myths, and history) but never feels overwhelming. Soul of Chaos is quite accessible, both in the narration itself and how everything is presented--and it takes some mastery to do that this well.
Third, I want to commend the author on the characters and the representation. You will not find any sexism or gender-driven violence here which is so refreshing. You have women who work as knights, women being high ranking officials, and no comments regarding their performance. It seems to be a queer-norm word, which is another plus. It is so good to finally find some high fantasy that does not rely on a mostly male cast.
Something to note is that because of this setting, some titles are gendered (e.g., duke and duchess) but others are not (e.g., baron, which should be baron and baroness); so, there is this female character which is introduced as Baron Shadowsong... and I had a bit of a metal glitch until I realised that she was a woman. It's not jarring, just be mindful of it.
Fourth, the magic system and the world setting is unique and compelling. Your average high fantasy would be quite medieval... but not this one. There is this mixture of priests that cast magic, priests that sing to do sorcery, alchemists... and then there is a whole group (I won't spoil much) that actually uses some fantasy-enabled technology. It gives the world a completely different uniqueness that isn't there in other books; additionally, you have people warring with greatswords, but also some troops wielding arquebuses, adding a compelling dynamic.
More importantly, the author balanced this magic/science swords/arquebuses contrast very well, making them work in such a perfect tandem. This "magic system" (using quotes because it's science, not magic) seems critical for the plot, it enables the plot basically, but I wouldn't say it is scifantasy. There are some explanations on those epistolary mini-chapters, but it is presented in an accessible way.
Nevertheless, it is not your traditional setting, and a very welcome difference.
Now, why four stars and not five? I found three things that I would have preferred to be different:
First, there is a particular character that speaks in another language; I believe the author uses French albeit it has an in-book name. The issue was that there were no footnotes with any translation (perhaps this was an ebook issue?); I'm not a French speaker, so I had this sensation of missing something. Granted, the other characters respond in a way in which a reader can somehow infer a bit of what the French line meant... but it was annoying and I'm quite sure I lost most of the meaning there--especially if there were any jokes or humour.
Second, given that combination of magic and science (which is just chef's kiss), I would have liked more description of the environment. The characters, their clothing, the armours, and the magic (or artefacts) are well described, but I wasn't really sure how to imagine the houses, or the structures, and the castles. I would have liked more details on the cities, on the scents of the places, on the colours and the people walking/passing by. Yhe book is short enough that a few more words in environment/architecture description could have painted a more vivid image for me.
Third, there were bits of un-tagged dialogue. Gratefully, the author does not plague the book with "said"s, but there are some sections of untagged dialogue with 2+ characters that I found myself re-reading just to ensure I got who-said-what correctly. A bit more aid (perhaps descriptions of the tone/posture) would be welcomed.
TL;DR: A great high-fantasy with tones of adventure, epic stakes, and a diverse cast of characters. Great world-building, an innovative magic system that enables the plot, and written in an digestible, easy-to-read way that flows naturally and is never overwhelming--even when the world is vast (both in terms of complexity and history). Recommended for fans of high fantasy, or for those wanting to give the genre a try.