I feel bad. There's a universe where Halls of the Arcanum could serve as the basis for a satisfying game of occult investigation, but that universe is, unfortunately, not The World of Darkness.
The Arcanum is an organization of scholars who seek the truth about the supernatural. They are patterned after the fin de siecle occult revival and operate on a kind of grab-bag hermeticism, with a special focus on alchemy and sacred texts. And that presents a prospective storyteller with an uncomfortable dilemma - are they good at what they do or not?
Because if they're good at what they do, then we kinda already have a group of guys exactly like that. They're called the Order of Hermes and there's no non-catty way to point that out. Aside from some Ars Magica-derived proper nouns, the organizations are 100% identical. One of the character templates is even called the "Hermetic scholar."
That made me laugh. It's like opening up a mockbuster on Christmas morning. "Hey Billy, I heard you were interested in joining an order of people who followed the teachings of Hermes Trismegistus. That's right! You're in the Arcanum!"
Not that the other horn of the dilemma is much better. If the Arcanum is not very good at what they do. If they are, in fact, the tightly-knit lodge of hermetic mystery-seekers that doesn't know what they're doing, so much so that the book straight-out says, "the collected tomes of [the Arcanum] are not brimming with accurate truths." Then what are they even for?
No, seriously, what are they for? They're woefully outmatched by everything they might run in to, but it's okay because they're only seeking knowledge and have no other goals for which they would want to put that knowledge to use. And in the end, they're going to get the bulk of it wrong anyway. This is pretty much the worst-case Ascension's Right Hand scenario.
My theory here is that the Arcanum predates all of Mage. It hasn't played much of a role in the game so far (how could it) and I'm pretty sure it's going to fade from memory sometime before Revised comes around, but previous books have been holding the door open for them. There have been a bunch of quick little references. Things like "The Arcanum would be interested in this" or "be careful or you might draw the attention of The Arcanum." It's possible that a group of normal occult investigators was always intended as a foil for Mage characters, and they simply failed to adapt their original idea when Mage evolved in a way that made the Arcanum redundant.
Ukss Contribution: An Arcanum narrator, bragging about his accomplishments, says "I have read the Poison Book." He doesn't explain what that means or why we should be impressed, and it never comes up again, but I thought it was pretty cool nonetheless. Good for you, nameless Arcanum guy. I will immortalize your accomplishment in Ukss.