Quest for the Secret Keeper

Delphi Keep is awash in activity, and for Ian, Theo, and Carl, their safe haven might be nearing its end. The Royal Navy has taken the keep to use as a hospital and the tunnels running under the keep and the castle are ideal to set up a central communications outpost for the approaching war. The earl is happy to help the effort, but now the keep is no longer safe for the orphans and they must be evacuated to his winter residence.
Ian, Theo, and Carl know that if they're sent away, they'll no longer be protected. But more important than their safety is deciphering the third prophecy. All clues point to a quest. The orphans don't know where they must go, but they know they must rescue the Secret Keeper.
To do that, however, they need to work out who this Secret Keeper is. And what, exactly are the secrets he's keeping?
From the Hardcover edition.