In Chapter 166, Devin told Bash that Mun and Lowls had left Cassomir to travel to this location because they believed it to be where the Necronomicon was being held.
Reflecting on the name, Pandora recalled the following:
“The Mysterium” is the modern, colloquial name for an ancient library originally known as the Hakkar Miniar, located in the city of Katheer. It was built over 3,000 years ago, in the early decades of Katheer’s founding. It had a ton of prominence and prestige in its earliest centuries as one of the Great Libraries of the Ancient World, and as an important place of study and instruction. But as time passed and Katheer grew and the world moved on, it eventually became just one among a long list of interesting libraries and institutes of study in a city that eventually grew to be one of the largest in the world. Today, the church of Nethys apparently oversees it and it has a reputation for being a quirky oddity, not a big attraction. “That funny little old building with the angel statues.”
Bash, however, recalled a little more (gleaned from spying on a private conversation between Mun and Lowls):
The specific sect of Nethysians that run the Mysterium are called the Stewards of the Vault, and they have dedicated their lives to protecting the world from knowledge that could destroy it. In fact, they took over control of this particular library because they found out that it was wildly more complicated, enchanted, and warded than anyone ever suspected. To such an extent that they turned it into a secure depository for rare and dangerous books. Books that need to disappear. Books that must never be opened. Or opened again.
At the heart of the Mysterium there is apparently an extradimensional space known only as the “Soul,” and the Stewards have placed every dangerous book that has fallen into their hands into the Soul, and dedicated their lives to maintaining and protecting it—all while keeping an extremely low profile about the site. They manage the Mysterium as though it were a run-of-the-mill quirky little library, and they let scholars peruse many of its floors. They like to give the impression of being out-of-touch scholars who just like their little old building and its dusty old books… but deep inside their building they keep a secret vault.
And no one knows exactly what’s in it.
Bash believed the site very likely to be warded against teleportation and divination.
In Chapter 168, Mun's Anomalous Friend told the party that Mun had headed into the Soul.
In Chapter 172, the party learned that the site was located on the west side of Katheer, in the city’s sprawling university district.
In Chapter 173, the party approached it for the first time. It was a blocky sandstone building: a perfect square around 100 feet by 100 feet, with vertical walls for about 15 feet or so and then atop that chunky base, a squat pyramid rose another 30 feet or so. Dirty, timeworn marble statues of angelic guardians stood at each of the building’s corners, armed with spears and shields. And the rest of the building, all of it, was covered with smiling angels carved in bas relief into the sandstone.
The only entrance to what seemed to be an otherwise completely sealed structure was a 10-foot-wide perfectly circular iron doorway in the center of the street-facing wall that gave the impression of a bank vault. It was fashioned into a dozen iron angels, severe and frowning, wrapped around each other as the flock of them enclosed the opening, and all were staring inward. In the two half-moon iron doors that sat inside the frame was one small keyhole, but it actually seemed like the purpose of the doors was not to keep people out, but to keep people in, because there were an intricate series of iron bars, levers, wheels, and chains installed over the doors that were clearly operable from the outside, not the inside.
Heavily warded with artifact-level magical protection. Guarded on the outside by a quartet of Stewards of the Vault.
In Chapter 173, the party learned further details about the Mysterium from Sister Thyrr:
- The building has numerous levels, and the ground floor is known as the University Level.
- The levels are connected by a series of valves/gates, and the method of opening them involves a complicated process.
- All levels are shielded from outside divination.
- All levels are shielded in such a way that teleportation effects cannot be used to enter or exit each level (though they do function inside each floor).
- There are artifacts known as the Guardian Mirrors on each floor that will call specific protectors when the wards are triggered.
- The Soul is an extradimensional space, well guarded by eternal protectors.
- The wards can only be deactivated deep in the heart of the complex.
- There is a magical item known as Folcom’s Key which can be used once per day to eject the protectors called by a specific Guardian Mirror.
- If the wards are not deactivated by the next New Moon, the site seals forever.
In Act IV Interlude Part 2, it was learned that the Guardian Mirrors were not summoning the specific protectors Elder Lythiin was expecting when four Hounds of Tindalos tore six of the Stewards to pieces on the University Level.
In Act IV Interlude Part 3, it was learned that the third level of the Mysterium was called the Cipher Obscura.
In Act IV Interlude Part 6 (as part of a revisiting of the initial conversation in Chapter 173), the party more about the Mysterium from Sister Thyrr:
- The second level is known as the Arcana. It contains several specific libraries: Practical Thaumaturgy, the Evocation Collection, the Occult Collection, and the Storehouse of Blasphemous Texts.
- The Cipher Obscura level also has several specific rooms: the Chamber of Whispered Secrets, the Worlds Beyond Collection, the Chamber of Calling & Binding, the Chamber of Creation, along with an alchemy lab and a study hall.
- The different levels of the structure are not located anywhere near each other in space. Rather, they are connected by magical Gates inside the valves, and could be located anywhere on the planet (or in the multiverse, conceivably). The valve Gates can only be opened by turning the heads of four specific angels on each floor and then manipulating the tongue hidden inside a specific angel face on the valve itself.
- Though teleportation effects will not cross the boundaries of the levels, summoning spells and effects do work.
- After the wards are triggered, none of the valve gates will open from “below” until the wards are turned off.
- There are eight total Guardian Mirrors in the complex, and they are supposed to be summoning various Lawful outsiders such as Axiomites, Inevitables, Aerial Servants, and Clockwork Mages. (However they are now clearly summoning Hounds of Tindalos, Proto-Shoggoths, etc instead.)
- Below the Cipher Obscura is a small room known as the Antechamber to the Soul. It contains a puzzle box that must be solved in order to gain access to the Soul, which is an extradimensional space. The Antechamber is guarded by four Soulbound Shells, believed to possibly be the four founding members of the Stewards of the Vault. The will attack anyone who attempts to open the puzzle box and fails.
- The Soul is protected by an Exscinder Angel known as the Keeper.
- The wards can be turned off in the Antechamber room by sticking one’s fingers into the sockets of the only angel in the chamber with no eyes.
- The entire complex is subject to random periodic haunts due to the traumas and violence experienced in the site over the centuries.
In Chapter 176, the main party entered the Mysterium for the first time.
In Chapter 185, Sister Shapari informed Pandora of Sister Thyrr's decision to seal up the Mysterium forever from the inside, and presumably end her own life.
The Mysterium sealed forever on 3rd of Gozran, 4719.
First Reference: Chapter 166
Other Notable References: Chapter 168, Chapter 172, Chapter 173, Act IV Interlude Part 1, Act IV Interlude Part 2, Act IV Interlude Part 3, Act IV Interlude Part 4, Act IV Interlude Part 5, Act IV Interlude Part 6, Chapter 176, Chapter 177, Chapter 178, Chapter 179, Chapter 180, Chapter 181, Chapter 182, Chapter 183, Chapter 184, Chapter 185, Chapter 192