The Harbinger of Death (The Equinora Chronicles)

When rotfrogs poison the wetlands in Westlands, Prism, the youngest unicorn, is sent by Aureana, Equinora's goddess, to overcome the invasion. But despite her golden wings, Prism is unable to fly due to her fear of heights. She makes the long treacherous journey across the mountains on foot, accompanied by Delsin, a spirit man and Rocky, a mortal horse. Prism falls in love with Rocky, but is tormented due to her desire for an immortal soulmate. By the time they arrive in Westlands, a blight has spread to all living things. When Prism confronts her grandsire, the guardian of the territory, a battle ensues between the older and younger generations of unicorns, causing devastating fire and floods. Prism must warn all the people and animals of the land to seek safety, at the same time as finding a cure for the plague. However, unless she can master her fear, she won't be able to save the whole of Equinora from becoming a desolate wasteland.