I loved Mythras, this is probably the best mechanically designed d100 rule sets in the double-zero gaming hobby. The original hardcover's font was too light and too small for my eyes, so the game never got played. The newer releases thankfully address that, and we have full ORC licensing of the rules, so it is a win-win for the community. The "Imperative" books are the ORC-licensed ones, and they update the key parts of the rules, make improvements, and bring the system forward with patches and fixes.
But the books still rely on the older volumes, and this is especially apparent in their Classic Fantasy line. We have the new ORC-licensed Fantasy Imperative, which instead of Bronze-age role-playing is the more familiar Middle Ages to Renaissance period. So, to play Classic Fantasy...
We need the original book, since the treasure tables for lower level items are here.
There is an expansion book, too, with the Classic Fantasy Expert Set.
Now, we are in the ORC era, so we need Classic Fantasy Imperative.
And there is a new book (in the ORC era) with more rules updates in the Classic Fantasy Unearthed Companion. I get it, they need to sell through inventory and they are a small company, and it is tough to survive unless you can ship already-printed products. You also want to update rules and get the licensing issues worked out for third-party support. The earlier books have font issues and are hard to read.
To be fair, if you are starting today, base everything on Classic Fantasy Imperative to start, and pull select things from the older books. Pull in fixes from Unearthed as you need them, but use Imperative as your "core rule book" for play. The Imperative books have solved the typeface issues, so they are solid, but not 100% complete.
Mythras is sort of a similar issue, where the older hardcover is still referenced, you have a number of expansions, and you have an ORC-licensed Imperative book that should serve as your "core book" while the other books are the ones to pull from.
I would love to see "one book to rules them all" from this system with a complete reference of all material. As it is, we have books under two licenses spread out over several books, with the latest ones being the definitive and patched sources. But this is still one of the best implementations of the d100 fantasy gaming rules, and many still say this is the set of rules to play by.
Black Lodge Games has recently put out The Cults of Zahak, a mature-themed (yet still tastefully done) book of evil cults and Diabolism for any d100 system, such as Mythras Imperative, Classic Fantasy Imperative, Runequest, Call of Cthulhu, or Basic Roleplaying. Yes, I had to edit the cover there since this is an all-ages blog, but the art is still not terribly salacious.
The book is sort of like the old Book of Vile Darkness, but done much more tastefully and presents evil cults and a sample campaign setting they exist in. This is a detailed "bad guy" book, explaining the evil cults in detail, their powers, how they corrupt and operate, and gives a wealth of detail and NPCs in the setting - some to trust, and many you wonder who's side they are on.
BoVD was more "emo evil" and trying to be extreme for shock value, so it was a bit juvenile "look at this" feeling for my tastes. Zahak is far more mature and far less salacious, and is more mature in a metal state of mind than it is presenting "raw nekkedness" for shock value. The art is mostly all public-domain classical pieces and beautiful, with some newer commissioned works of equal skill and beauty.
I have not seen an "intrigue and darkness" setting done so well, with plenty of figures to interact with, random encounters they can appear in, and the players need to figure out why the terrible things happening happened and who is responsible for them. The cultists can employ plenty of misdirection, so they could lay blame on an innocent, so careless players who jump at the first evidence could be acting on the wrong information and punishing the wrong person.
A terrible, yet strange, accident could happen (like something out of the Omen movies), and the players could investigate, the inquisitors could be there, innocents could be blamed by cultists trying to misdirect, seemingly innocent people could be feeding both sides false information, cultists will be setting up more accidents or turn monsters loose on the investigators, and there could be corruption within the ranks of the inquisitors trying to confuse the entire situation.
The players need to be crafty, sneaky, use their heads, make notes, find inconsistencies in statements, and use their heads to figure out who is on what side, who can be trusted, who is being falsely accused, and who the guilty are. All while trying to survive being attacked and set back every step of the way.
For very character-driven mysteries in a fantasy setting against cults of darkness, there is no better book than this. this is almost like a book that could setup a very cinematic game world and adventure that feels like a movie. This is not your ordinary "5E adventure" with keyed locations and CR-appropriate combat encounters. This is a thinking person's book filled with darkness, corruption, and intrigue.
This is great fantasy role-playing campaign material, and provides a unique campaign rife with opportunities for interaction, investigation, and combat without needing a lot of the "fantasy monster and dungeon tropes" - but you could still integrate those for a more fantasy flavor. Rarely have I seen an evil faction book done so well and laid out.