All Things in the Shadows — B.D. Messick

Seventeen-year-old Eve's world is about to come crashing down. When the mysterious Kateri enters her life, Eve is introduced to the hidden realm of the Shayds; an army of warriors who protect the world of light from the forces of evil. With Kateri's help, Eve will discover how much of a difference one person can make, and that there are more things in the shadows than we know, but this knowledge will come at a terrible price.