The most significant change in the revised and expanded version of Mysterium (if you have bought the game, check your drivethru library for the new files!) is that made to the rules for mental stress. Instead of the five meters nicked from UA there are now simply two: Mental Stress and Paranormality. This is kind of like Stability and Sanity in ToC, where Mental Stress is about all kinds of mundane pressure and traumas, while Paranormality deals with shocks to the character’s sense of reality. The mundane stress can cause a PC to become indifferent, distancing themself from their emotions, or suffer mental disorders. The stress caused by high strangeness can cause the same kind of difficulties, but is also part of an initiation into cosmic mysteries. When a PC reaches ten hardened notches or five failed ones in Paranormality they enter into a crisis since their concepts of existence and reality no longer function as useful maps to the world of their experiences.
This state of crisis is called the Chapel Perilous, borrowing a term that Robert Anton Wilson used in his autobiographical account of high strangeness, Cosmic Trigger, and which originally comes from Sir Thomas Malory’s Le Morte d’Arthur. For a deep dive into the concept of the Chapel Perilous, check out this essay by Matt Cardin: https://www.teemingbrain.com/2012/08/13/initiation-by-nightmare-cosmic-horror-and-chapel-perilous/
Basically, the PC can exit the Chapel Perilous either by embracing the Weird, Strange and Eerie at the heart of existence, or by loosing themself in delusions where nothing or everything is magical. Now, this will naturally look quite different depending on the particular cosmology and mythos of the campaign, and the specific experiences of the PC. Mysterium is a weird fiction game about people having close encounters with unexplained anomalous phenomena, and these rules are the scaffolds for constructing the scene where we play to find out how this encounter unfolds.
Caverns Measureless to Man 