AU. After James Potter has deliberately revealed Severus Snape creature inheritance, Snape is rated as dangerous, and has to leave Hogwarts. Years later Snape is the only person who can save the life of the pregnant Lily Potter. Snape helps, but for a price: He will marry a Potter child before her/his 20th birthday. James, fearing for the lives of both his wife an unborn child, agrees. When Harry finds out about the agreement on his 16th birthday, he's beside himself with rage.