The Sorcerer in the North (Ranger's Apprentice, #5)

Five years have passed since the Skandians and the Araluans made their treaty, and Will has finally become a Ranger, with his own fief to look after. He soon learns that even sleepy little island fiefs have problems to keep him on his toes.Then he and his old friend Alyss are thrown into a terrifying new adventure, investigating the truth behind rumours of sorcery in a remote northern fief. As he stands in Grimsdell Forest, hearing eerie voices and seeing the horrific figure of the Night Warrior looming in the fog, Will must ask himself one question: is there a rational explanation... or does sorcery really exist?