There are minor errors here and there, but given it's 500 pages of stats that's perfectly understandable. This book is a wellspring of information that has given me so many ideas for encounters. I love the way you handled certain spell-like abilities by assigning them an equivalent spell level for the purposes of counterspell and dispel magic, and I love the monster design overall. 10/10 wonderful book.

Hektula's Khyber Codex has all the hallmarks of a labor of love and passion. There is so much content considering the comparitively small price. Core boss monsters, like the Daelkyr, are given intricate and wonderfully creative statblocks that any DM would love to control (with a particular shout-out to Valaara the Crawling Queen and her psudo-stace changer design which is a delight). The various demiplanes are well thought out, offering effects, rewards, dangers, encounters, etc. for many locations previously named in Eberron lore, but never fleshed out, or wholely original works that add so much to the setting. This is an Eberron-faced book primarily and I recomend it for that, but if you want creative extraplanar adventures there are many takeaways from this book that can be tinkered with to be used anywhere. Fantastic work and well done.

Fluff: I have zero complaints about the fluff. The lore details this book adds is astounding and well thought out. Careful attention was placed in ensuring each nation fealt distinct in their views towards Tieflings and how they would be integrated into society, even if it's a tad more accepting than I would normally run it, the plot hooks and character suggestions can still be implemented with little effort, and the possibilities for character backstories are amazing.

Crunch: My feelings on this front are a tad mixed, certain Tiefling Origins that gain a bonus to mental stats other than charimsa, such as Dark Six tieflings which gain a bonus to wisdom, but still use charisma for their racial casting. This is partially understandable as it's using the base Tiefling race/subrace combo which gets a +2 to charisma, but I still wish they took inspiration from the Variant tieflings such as Hellfire, where base gives a +1 and you choose a +2, and then add the option to use the mental stat gained as the spellcasting ability, but I can't fault the creator for sticking to the fundamental design for tieflings. Additionally, I will note that I was disapointed that Sakah Tieflings only got a single subrace option tied to a fear aura instead of casting. The book offers the suggestion that Sakah are rare mutations born due to proximity to an Overlord prison, but does nothing to capitalize on this mechanically. Overlords are as interesting and diverse, and representing all of that as just a fear aura was... unfortunate. Somewhat understandable given the Overlords connection to fear as a whole, but still dissapointing. Beyond that I will note that some racial spells are better than others for subraces, but that's to be expected. However, barring the specific examples I listed, all options provided are interesting and flavorful.

Overall: Considering Tiefling Treatise is, primarily, a lore book that offers so many wonderful suggestions to players and DMs alike it's a wonderful product. As a book delivering new racial options it's great, and offers many interesting options for the majority of Tiefling origins provided, barring some small dissapointments that I can overlook. Fantastic product at a fantastic price.

Extra: Love the art.

Edit: To clarify I was not expecting every Overlord to be represented with their own subrace with the Sakah, as there are 30 Overlords and that would be a tad much for the scope of this project. I was just hoping for the most prominant ones, such as Sul, Rak, and Bel to be represented in some form.

This book is everything I wished I got in Van Richten's Guide to Ravenloft when it came to exploring a Dread Domain and the horrible implications within. There's so much material to work with, proper stats for the Darklord Herself (The Queen Will Save Us) as well as some horrifying new monsters, all while tying it into my favorite setting witout interfering with it's cosmology. My one complaint is that, although I love the new Artificer Specialization, the Mastermaker, most of it's key abilities don't come till level 9, making it feel like a much weaker Armorer until that point, but that's a small complaint when there's just so much good content here. Worth every penny.

Edit: Errata has fixed Mastermaker, upping it's damage die to d10 and allowing you to give it a feature at level 3. Fixed any issue I had with it. 10/10 would give Monk what-for with it again.