Mysterium

Mysterium is the Episode: Heresy system that holds Faded Scrolls, which strengthen the boons collected in The Nether. It is opened at the Shaping Slab in Eris Morn's Apartment in The Last City, and can be checked there at any time.

Collecting a given boon in The Nether raises a count for that boon. Once enough have been collected, its Rare Faded Scroll unlocks, and future runs give that boon 2 stacks per pickup. Legendary Faded Scrolls require other achievements, such as unlocking Pinnacle Secrets or uncovering collectibles.

Sources describe the scroll rewards in slightly different terms, so check the in-game text for exact values.