Mysteries & Mystagogues is the Game Master’s guide for Chthonic Codex. It’s not really structured as a typical GM book. M&M is really an improv tool for running a Chthonic Codex game. This is a list of the contents:
There are two d66 adventure hook tables: one is a traditional event-shenanigan table, the other is structured as chains of events and consequences. A third table is simply a list of missing and urgently needed magical ingredients, so that your players can go bumble around looking for them. Often the PCs’ Master (their teacher) is central to the hook. But it might not. Masters are a mixture of stick and carrot, and usually are PCs’ patrons.
Then there are accomplishments: they are usable both in AFG and your Old School game of choice, and they’re supposed to be discovered in game, and when completed they give perks to characters: for example extra mana, special powers or a level (there are no XP in AFG). They work as hooks too.
Then there is what I would have called MYSTATRON had I been smart enough. It’s a generator of mysteric initiation rituals. It’s structured as a collection of nine d66 tables. It’s followed by a section of powers, usually gained trough initiation, but also in other ways mentioned in the book. MYSTATRON gives your players a structured serie of things to be found and done, and a guarantee of extra powers at the end. If they don’t die complete the initiation. A few times I was totally stuck with players rummaging in a lost library and, a quick roll of a few d66 on the first pages of MYSTATRON will give them a weird ritual to be carried out, with the relevant preparations. Basically, a few sessions of adventuring.
Then there’s a part of Laws of Reality. You know when you granny told you never let albinos spill salt, or never to lose goats at night, or never dance in a graveyard? She was right, and this section tells you what happens when you do that. It’s another way to do “magic” that is not magic. It works in parallel with other rules, and not only for spellcasters, and they are mostly based on the player’s knowledge, as they do not necessarily require magic powers! These should be rolled before the campaign starts, but don’t sweat it if you don’t.
Then there are four pages on the ritual of the Apotheosis of the Grand Sorcerer of the Valley of Fire.
CHTHONOTRON follows, with an extra section about pointcrawling in caves and canyons. It tells you how to make a chthonic pointcrawl to use in the game and populate it with catacombs, grottoes, chasms home to chthonic cods, forgotten squid settlement and their lost artifacts, shrines, monasteries, and goats. Lots of goats. GOOOOOATS.
After that, a section on the famous Hypogean treasures and curios, including the strange powers of the devouring idols. Not that any of them, like the Butyrous Sarcomancer or The Enemy ofTruth and Beauty, can do nasty things to your campaign. Some scrolls are thrown in the section too, because gaining knowledge is the biggest preoccupation characters should have. Well, beside staying alive. I mean, if their self-preservation is stronger than their thirst for knowledge, why would they study magic in a school under a desert scoured by flames? The scrolls are in because you need something interesting to put in scrolls, and you need a random table for it with results that are not “a spell” or “a treasure map”.
A small section on the value of truth follows. Tables with random names for characters and chthonic gods close the book. I added the tables because you need to give names for, you know, all these unexpected and unnamed NPCs and gods. It sucks when you can’t come up with a name for the assistant pharmacist and the god that lives in the chasm by the standing stones on that island, and I wanted the names to sound right for the campaign.
Lost Pages 
