Selûnite Journal contains a hint for opening a door to the Underdark.
An aged journal.
Properties
Author: Church of Selûne
Rarity: Common
Weight: 0.5 kg / 1 lb
Price: 14 gp 
UID
S_GOB_TempleAccess_LorebookUUID
283d7eff-f6e5-401f-852e-7ad7fb728b88
Where to find
- Cabinet in Gut's quarters in the Defiled Temple X: 386 Y: 8
Text
[Most of the pages in this aged journal have been hastily torn out, perhaps for kindling.]
Thus the interlocking circles will bring the full moons to match the stars, while casting darkness where it belongs at the bottom. That chasm to the Underdark will stay sealed.
I confess the design is not foolproof to outsiders, but I had to sacrifice complexity for material resilience. I've always wondered why Selûne took me from the Hall of Wonders to serve at this temple as a priestess. Perhaps this was the reason.