The Mask and the Sorceress — Dennis Jones

For seventeen centuries a primal sorcery deemed too dangerous for use by humankind has been locked away by a powerful spell. But the Deep Magic is chained no more, and war again threatens a royal house. Theatana -- the insaneregal sister exiled for her treachery -- has escaped her prison, discovering within herself seeds of the blackest power which will help her ascend the throne she believes rightfully hers. And now she prepares to launch a merciless assault upon her own homeland...and her blood kin.
But there is one determined to thwart her: Ilarion, the true heir, who will pursue his mad aunt to the very point where the world itself stands on the brink of catastrophe. It is here where a tumultuous contest must be waged -- and a devastating power will be revealed.