The Dragon Child - The Dragon Child Series, #1

One day all darkshall reach full power. All light will fadewhile the dark devours. All shall be lostuntil the child is born, The one for whomthe Dragons mourn. When Lucy Clare goes on a school trip to Scotland without her parents' permission, the last thing she expects to find out is that she's the key in an age-old prophecy. The Morrigan, an ancient Celtic goddess, is coming and Lucy is the only one who can stop her from destroying the worlds.
When the Morrigan accidentally kidnaps Lucy's best friend instead, saving the multiverse becomes personal. Lucy must embark on a journey that will bring her face to face with evil. But destiny is not a paved path and Lucy must fight through a world of darkness, magic and old gods before she can rise as the Dragon Child