At the conclusion of Scarlett's big battle in the Water Gateway, Death will make in a belated appearance and dump more information on his cursed daughter. The Black Grimoire contains a dark ritual that can destroy the world.
As soon as Scarlett reaches the Outer City, if she asks about Sophistos or books (and sometimes even if she doesn't), people (guards, Rico, Aeris) will suggest she go see a fellow named Nox, who lives in the Backstreets area of the Outer City.
When Scarlett reaches Nox's home (enter the Backstreets and go up some wooden stairs to a landing), she'll find the door locked, but a "neighbor" named Rat will tell her that Nox went to the Old Temple. When Scarlett gets to the Temple, a guard will warn her it seems to be cursed. Inside, she'll see Nox attempting some sort of ritual from a book. But, things seem to go awry, and Scarlett will get herself locked into the temple with some Lectors.
After Scarlett has dealt with the Lectors, she should use the Gateway Idol. There, Benedict we'll give her the mental skill "Eye of the Lost". Back in the temple, Scarlett will still find herself locked in. But now, if she uses The Passage, she'll be able to see the ghosts of two old friends, Don and Nesto, ready to help her with their mechanical skills by showing her how to pick the lock. (Go ahead and have them help her pick the lock on the chest in the temple foyer on her way out, too.)
Scarlett can now return to Nox's home, where she will find him still being harassed by Lectors. after she deals with them, she can finally talk to Nox.
Nox will give Scarlett the Trust to Be Earned quest, which she will need to complete next, in order to get him to tell her the information she needs about Sophistos and the Black Grimoire. (Apparently, saving him from a couple of Lectors wasn't a sufficient demonstration of her sincerity.) Getting this next quest completes this one.
- Nightwatch Troubles - Scarlett will need Nox to teach her some spell (mental skill) she can use to kill the Flying Beasts.