His Cure for Magic — M.R. Forbes

Magic is a disease. Magic can be cured. He has it, they need it, or thousands more will die. In the second book of the Tears of Blood series, Silas and Eryn find themselves running out of time and with only one hope: an ancient map discovered by chance, revealing the location of a place thought only to exist in legend. The Dark. A place of nightmares. A place of death. A place of magic gone horribly wrong. A valley perpetually shrouded in thick clouds, where the very air instills the deepest fear, and monsters lurk in every crevice. It is a place none would visit by choice, but for the Silas and Eryn there is no choice. They must descend into its darkness and uncover the secrets hidden within. If they fail, Eryn and the magicians like her will die, taking all hope of a free Empire with them. They think they are searching for the cure for magic. Nothing will prepare them for what they find instead.