This is a Fallen London exceptional story, which is something accessible only to members or those who pay extra money. It follows the mechanics of Fallen London but is completable in a few hours of time (or less if you pay for more actions).
This story deals with the Presbyterate, an underground empire that lives near the Mountain of life. They had a colony founded by separatists who had a mythical chalice that seems like a parallel to the Holy Grail. You encounter an archaeologist who leads you to the sea in search of this chalice.
This story gives you some pretty good agency as there are multiple directions you can influence the outcome. It has some rich and vivid imagery in the archaeological digs that does a good job of both being cool and showing the differences between the archaeologist's idealism and the more grey reality.
This one was pretty good. It didn't stand out as one I'll always remember but it was interesting enough that I used some action refreshes on it.