This article contains plot details from Legacy.
Forgotten Secrets are memories so dangerous that Councillors have to purge them from their minds and store them in caches so their guilt doesn't shatter their sanity. The caches are stored in the void. Until a secret document was given to Sophie by Lady Gisela, Vespera was part of a Forgotten Secret. As a result, only the guards that tended to her in prison in Lumenaria knew of her existence and who she was.
Councillor Oralie gave Sophie the late Councillor Kenric's cache just before Sophie and her friends left the Lost Cities to join the Black Swan. Kenric's cache contains seven Forgotten Secrets. Fintan's cache contains nine Forgotten Secrets, although it is later discovered that both of the caches are fake.
At the end of Legacy, Oralie tells Sophie her theory that her cache's sole Forgotten Secret has to do with stellarlune. The two of them agree to work together on opening the cache.