Mists of Darkness — M.J. Evans

Mists of Darkness is a 2014 Gold Medal winner of the Mom's Choice Awards!
Having braved the evil powers in the Dark Kingdom and rescued a lost soul in Behind the Mist, the first book of the Mist Trilogy, Nick and Jazz proved themselves worthy to become members of the Legion of the Unicorn. In a ceremony passed down through the ages, Jazz is endowed with a glowing, amber-colored unicorn horn and a new name: Lazari. Nick becomes his partner as the first-ever unicorn rider.
Again and again, the mist of darkness descends at horse venues around the world. Horses, of all types, breeds, and sizes, disappear in its oppressive gloom. Urijah, the Lord of Celestia, and his Council of the Twelve Ancients watch helplessly, never knowing where it will strike next.
Will the Legion of the Unicorn be able to stop him? Read Mists of Darkness-Book Two of the Mist Trilogy to discover what happens to Nick and Lazari in this exciting continuation of the acclaimed fantasy, Behind the Mist.