A covert wartime cipher draws the Archivian Museum’s Field Core from a locked room in Cambridge to Cartarra’s breathing sea caves. In a forbidden tomb, a wrapped relic offers wonder, and demands a choice no map can solve.

In the mist-laden swamplands of Cartarra, the Field Core races against rivals and the treacherous unknown to recover a fragment of a legendary medieval chalice—only to find that not all allies, or enemies, are as they seem.