The Magician by Michael Scott (2008) Delacorte Press

Being the second volume of The Secrets of the Immortal Nicholas Flamel.  Once in Paris, the twins and Flamel find themselves in a church.  Because of the modern age and people who like to destroy what is not locked down, there are locks and alarms on the church.  The police arrive along with another immortal, Machiavelli, who is now the head of France’s secret service.  He also supports the Dark Lords and thinks he has trapped the trio.  But Sophie has the ability to use the wind and she creates a fog so thick, the police cannot see her or her brother or Flamel.  They escape, or so they think, to the Compte de San Germain, who it turns out is married to Joan of Arc.  When Dee killed Yggdrasil, he loosed the Nidhogg from its roots.  Machiavelli has called upon the Valkyries, who hate Skathatch because she nearly wiped them all out.  Machiavelli’s personal assistant, chauffer, and bodyguard, Nodthose, also hates Skathatch for a similar reason.  He is the last of his kind.  But Machiavelli holds him back.  What they don’t know is Flamel has returned to his old house in Paris, which still stands, and retrieve Clarent, the twin to Excalibur, and the sword used by Mordred to kill Arthur.  Germain teaches Sophie Fire Magic.  Germain’s house is attacked and the Valkyries loose Nidhogg.  Sophie manages to freeze two of the Valkyries, but Nidhogg captures Skathatch and leaves for the river.  Josh, using Clarent, manages to turn the monster to stone and Skathatch is released just before the monster sinks into the Seine.  But before they all can catch their breath, Nodthose leaps from the river and grabs Skathatch.  Flamel and the twins leave for London.