"Skulkin' around Knockturn Alley, I dunno -- dodgy place, Harry --don' want no one ter see yeh down there --"
     --  Rubeus Hagrid

Knockturn Alley is a dingy alleyway where the shops are devoted to Dark Arts items. The alley is connected to Diagon Alley near Gringotts,

Borgin & Burkes is the largest establishment in Knockturn Alley. It sells Dark Arts magic items, some of which are very dangerous. The proprietor, Mr. Borgin, buys as well as sells all manner of strange and sinister things. There are stores selling shrunken heads, live gigantic black spiders, and poisonous candles. Street vendors include an old witch with a tray full of whole human fingernails. And somewhere in the alley, according to Hagrid, someone sells Flesh-Eating Slug Repellent. (CS4)

Wizard's Atlas: Diagon Alley