Sisters of Sword and Song by Rebecca Ross (U.K. Edition)

Evadne has always looked up to her beloved older sister. Halcyon - a skilled, strong soldier in service to the queen. But when Halcyon appears in Evadne's bedroom late one night, bloodstained and terrified. Evadne must come to her aid. She learns that Halcyon is part of a plot to topple a treacherous mage who would overthrow the queen. All that stands between the usurper and the throne is a legendary relic: the crown of Acantha. Now. Evadne must finish what Halcyon began. As a web of intrigue and deception tightens around her. Evadne travels to the depths of the underworld with Damon, a young mage for whom Evadne's feelings grow stronger by the day, in search of the crown. And if she fails, she will lose Halcyon forever...