Dragon Blade (The Cycle of Oak, Yew, Ash, and Rowan)

is book #4 of The Cycle of Oak, Yew, Ash, and Rowan.
==Book Description==
Over a year has passed since the defeat of the Great Foulness, and the ravaged earth has begun to heal. Lost for generations, the signet rings of the Four Great Houses---Oak, Yew, Ash, and Rowan---have been restored to their rightful heirs. And Ashen NordornQueen, mistress of the Land of Ever Snow, looks forward to a life of peace and happiness with her beloved husband and their newborn son---only to learn that an ancient evil still threatens all that she holds dear.
The Mother Ice Dragon, the fearsome progenitor of her deadly breed, has awakened from slumber to menace the world anew. Legend holds that only the Dragon Blade, forged from the scales of her vanquished mate, can slay the deadly female dragon, but the Dragon Blade has been lost for ages.
As Ashen embarks on a perilous quest to find the mystic sword, she leaves her castle and homeland in the care of her closest friends, including Rannore, Lady of the Rowan, who soon faces danger of a different sort....
Dragon Blade continues the saga begun in To the King a Daughter and continued in Knight or Knave and A Crown Disowned.
(Description from SashaMiller.com)