Book Four of Merlin's Descendants.
Deirdre Kirkwood, the bastard child of Griffin, a Catholic priest, and demon-infested Roanna Douglas, has been raised with the children of Griffin's twin, Donovan. Each has inherited a powerful magical talent and a wolfhound familiar. Any of them could become the next Merlin of Britain.
Deirdre's quest to know anything about her father leads to a confrontation with El Lobison the Master of all Werewolves. No one is safe from him.
In and out of royal courts, the three cousins build a network of spies who are the only ones who can counter the weather, the werewolves, and the Spanish as England faces the relentless Spanish Armada.