You must research about the spider lady in the royal library to progress with that path. To gain access to the library, you must be a noble, and you can ask about it to the emperor when you visit the castle (so, once you go back after you first get nominated as a noble). To convince the emperor, you can either be part of his harem, have a good relationship with him, or have completed the quest related to the smuggled mana crystals.
The ritual that can only happen during the full moon is a different one, so you probably got confused.
I hope to add another "advanced" library soon, so people will be able to complete this quest without having to "suck it up" to the emperor.