Eye of the Falcon (Gods and Warriors Book 3) Penguin Random House Children's UK

Winter has been colder than anyone can remember, and there is no spring. The eruption of Thalakrea has shrouded the sky in ash, and blotted out the Sun. Fate brings Hylas to the island of Keftiu. He is desperate to find his friends: Pirra, the daughter of the High Priestess, and Havoc the lion cub.