Skandar and the Phantom Rider by A.f. Steadman

Skandar's adventure among the warrior unicorns continues in this spectacular sequel to the instant New York Times and international bestseller Skandar and the Unicorn Thief. Skandar Smith has achieved his dream- to train as a unicorn rider. But as Skandar and his friends enter their second year at the Eyrie, a new threat arises. Immortal wild unicorns are somehow being killed, a prophecy warns of terrible danger, and elemental destruction begins to ravage the Island. Meanwhile, Skandar's sister, Kenna, longs to join him, and Skandar is determined to help her, no matter what. As the storm gathers, can Skandar find the key to stop the Island tearing itself apart before it's too late for them all.