The spell that binds them can only be broken with an incantation found in the Codex Merlini, an ancient grimoire. The Codex's location has been lost in the present day, so Cassie will have to seek it out in the only place it can still be found--the past.

But Cassie soon realizes the Codex has been lost for a reason. The book is rumored to contain some seriously dangerous spells, and retrieving it may help Cassie to deal with Mircea, but it could also endanger the world... -- From Amazon.com Product Description (less)