The darkening - Yvonne Navarro

Wrestling with her increasing addiction to magic, Willow is devastated when her inability to stop using her power results in Tara leaving her. Far worse is to come however when no sooner are she and Tara reconciled than Tara is shot and killed by a stray bullet which had been meant for Buffy. Out of her mind with grief, Willow opens herself to the full power of her magic -- but magic of the darkest possible kind. Determined to revenge herself on those responsible for her lover's death, Willow begins her nightmare descent into evil. In the woods outside Sunnydale she exacts a terrible price from the captive Warren, who fired the fatal bullet -- she flays him alive; Buffy, Xander and Anya arriving too late to stop her.
But the death of Warren is not enough for Willow. Now her friends can only watch as the juiced-up witch sets off on a trail of vengeance and magic-gathering to prepare a spell that will bring Tara back to her. Whoever gets in her way is going to regret it -- including the Slayer, still hoping to save her best friend...