A long time ago, whenever the Oak King died, his followers went to war over the succession. War spread to their neighbors the Holly, and soon engulfed the entire Wood. Tired of the fighting over the throne, the wise came up with a perfect solution: the Oak King would be killed by a Holly Champion, and the Oak warrior who killed the Champion would become the next King. For generations Holly boys vied with one another to be chosen as the Tacaí, a warrior who would study to become any Cuilinn, the Killing Tree. The Cuilinn was beyond reproach, and had powers beyond the other Holly. But his whole life revolved around one task: to kill the Oak King, a powerful shape-shifter, almost twice the size of a normal Holly. To do this, the Holly bred berserks, warriors who would take on the Fury and stop at nothing to kill the King. Into this world is born Tréan, a half-breed, tormented and teased by his full-blood cousins, pushed by his uncle and father to win the Contest. Yet Tréan is disgusted by a contest he cannot win. When he runs away, Tréan is found by another half-breed who becomes his only friend. There is only one catch: his new friend is the son of the current Oak King! And his friend needs Tréan to become the next Cuilinn, the next Killing Tree.