The Enchantress by Michael Scott (2012) Delacorte Press 

The last volume in the Secrets of Nicolas Flamel, in which everything comes to the ending and the beginning of the circle of time.  The twins, Josh and Sophie find themselves, Virginia Dare and Dr John Dee 10,000 years back in time, while the Flamels and their allies, some unlikely, are in the present and are fighting to keep the Elders from releasing monsters on the city of San Francisco.  The twin's Aunt Agnes turns out to be She Who Watches, the first clay statue brought to life by Prometheus and she is the one who taught Scathatch and her sister Aiofe, to fight.  She is virtually invincible and she goes to help Prometheus