Harry Potter and the Half-Blood Prince

Lord Voldemort has returned to power, and his wrath has been felt in both the Muggle and Wizarding worlds. Severus Snape, long considered an enemy of Voldemort and a member of Dumbledore's anti-Voldemort coalition, the Order of the Phoenix, meets with Narcissa Malfoy, mother of Draco and wife of Lucius, an imprisoned Death Eater. Snape makes an Unbreakable Vow to Narcissa, promising to protect her son, Draco.