Light in the Dark

It's the summer before Harry Potter's 6th year at Hogwarts. Voldemort is rising. Dumbledore calls on an unknown ally to help the Order with a mission. In order to do so, she is tasked to work closely with the mysterious Severus Snape. The two become close as the year progresses, but not all is as it seems. Darkness is spreading quickly. Will the Light succeed? Or will the Dark win?