Unlike other foxes that roamed the woods of southern Ionia, Ahri had always felt a strange connection to the magical world around her; a connection that was somehow incomplete. Deep inside, she felt the skin she had been born into was an ill fit for her and dreamt of one day becoming human. Her goal seemed forever out of reach, until she happened upon the wake of a human battle. It was a grisly scene, the land obscured by the forms of wounded and dying soldiers. She felt drawn to one: a robed man encircled by a waning field of magic, his life quickly slipping away. She approached him and something deep inside of her triggered, reaching out to the man in a way she couldn't understand. His life essence poured into her, carried on invisible strands of magic. The sensation was intoxicating and overwhelming. As her reverie faded, she was delighted to discover that she had changed. Her sleek white fur had receded and her body was long and lithe - the shape of the humans who lay scattered about her.
However, though she appeared human, she knew that in truth the transformation was incomplete. A cunning creature, she adapted herself to the customs of human society and used her profound gift of beauty to attract unsuspecting men. She could consume their life essences when they were under the spell of her seductive charms. Feeding on their desires brought her closer to her dream, but as she took more lives, a strange sense of regret began to well within her. She had reservations about actions which never troubled her as a fox. She realized that she could not overcome the pangs of her evolving morality. In search of a solution, Ahri found the Institute of War, home of the most gifted mages on Runeterra. They offered her a chance to attain her humanity without further harm through service in the League of Legends.
Source: The Game Website.