Dark Sun: Chronicles of Athas, Book 04 - Cinnabar Shadows

Water. On Athas, it is life, it is currency, it is power. The watery caverns beneath Urik have become a haven for political refugees and runaway slaves, a place safe from the ravages of Athas and her defilers. Until one such defiler brings destruction to the haven. Mahtra, who fled sunlit Urik for the dark undercity, must now emerge to set all to right. But it isn't as easy as that. Her quest leads her to the palace of a sorcerer-king, to an ancient druidic grove, and beyond….