Burning Shadow (Gods and Warriors Book 2) Penguin Random House Children's UK

Hylas the Outsider is captured by slavers. Set to work in the terrible underground mines of Thalakrea, he learns to his horror that he's now closer than ever to his murderous enemies, the Crows. He has to escape before they find out he's here. Pirra, the daughter of the High Priestess, is also on the run....