The Arcanum is an ancient book that was written in the 15th century by an astrologer/astronomer Basileus while he was under demonic possession. It contains spells for capturing and summoning ghosts, profiles on the thirteen ghosts of The Black Zodiac and blueprints for creating a mansion-like machine with which to open and gain control over the Ocularis Infernum.
Over the centuries, many people died in the quest of trying to find and obtain The Arcanum before it eventually fell into the hands of Kalina Oretzia of the Spirit Reclamation Business. She and Cyrus Kriticos, whom she serves in secret, used the blueprints in the book to perfectly reconstruct Basileus' Machine, and to gather twelve of the thirteen ghosts needed to power and activate the machine.
The Arcanum was most likely destroyed, either when it and Kalina were trapped between two closing glass walls in the machine by Cyrus, or otherwise, when the machine itself exploded, destroying the house and releasing the ghosts.
- At some point during their possession of the book, Cyrus and Kalina used the book to contain a recording of the Latin spells needed to summon the ghosts.