Fuller, J: TRIALS OF THE INNERMOST

A cataclysm in Etherea's past stopped its rotation, leaving three distinct realms: one of light, one of dark, and one of twilight. Years of war followed until the establishment of the Trials of the Innermost.
A new set of six Truthseekers is chosen for this quest of learning and cooperation, but the fragile peace created by the Trials is crumbling even as they begin their first task. The group consists of each realm's best and brightest, and their talents range from magic channeled through music to explosive crystals. They will need all their unique abilities to survive the Trials, which take them through scorched wastes, a wall of storms, and the shadowed Fellwood.
Along their journey, a shared lost past is rediscovered, forcing the Truthseekers to question all they believed. As they grapple with their new reality, a deadly secret is unveiled. A fragment of an alien force called the Penumbra has possessed one of their own, granting him preternatural abilities. One realm seeks to control the entity, but the Penumbra has other plans. If it succeeds in reuniting its broken pieces, it will gain the power to consume Etherea's abundant life force. The Truthseekers must overcome their differences and work together before they lose their friend-and their world-to the Penumbra's hunger.