What is the introduction of Harry Potter and the Half-Blood Prince?

Answer 1

The introduction describes the Black sisters Narcissa Malfoy and Bellatrix Lestrange visiting Severus Snape's house. Voldemort has ordered Draco Malfoy, Narcissa's son, to kill Albus Dumbledore, a task which he knows is impossible. It is a punishment for Lucius Malfoy, Draco's father, failing to collect the prophecy from the Department of Mysteries in the Ministry of Magic at the end of Harry Potter and the Order of the Phoenix. The conversation between Narcissa, Bellatrix and Severus begins with Narcissa explaining the situation to Severus. Bellatrix cuts in accusing Severus of disloyalty. She then lists questions that she believes Severus cannot answer, which would indicate his guilt. However, he answers all of them quite easily, so she is left without proof. Narcissa asks him to take care of Draco and guide him in his task, and to complete it if Draco cannot. Severus agrees, but Bellatrix asks how they can be sure he will truly do so. Severus then makes an Unbreakable Vow to Narcissa.