Places
Knockturn Alley
Knockturn Alley is the darker shopping street that branches off Diagon Alley, known for Dark artifacts. Harry lands in it by a bad Floo connection and stumbles into Borgin and Burkes, where Draco Malfoy is selling poisons. Tom Riddle once worked in that shop.
First mention
Harry Potter and the Chamber of SecretsChapter: At Flourish and Blotts
Harry says the Floo destination unclearly and steps out into Borgin and Burkes instead of the alley.