Charlie Bone and the Wilderness Wolf
From Charlie Bone Wiki
Charlie Bone and the Wilderness Wolf is the sixth novel in the Charlie Bone series. The story starts with Charlie Bone's Uncle Paton and Charlie. In this novel, Charlie and his friend set out to rescue their new friend Asa Pike, who switched sides near the end of Charlie Bone and the Hidden King. The Bloors are angry at Asa and lock him up in a dark prison in the forest next to Bloor's Academy. In the darkness, Asa turns into the dark beast which he is forced into if there is no sunlight. Asa howls for ages, until Billy (who can understand animal tongues) hears him calling for help, but cannot recognise the voice.
