Places
Spinner's End
The street where Snape lives in a house near a disused mill. Narcissa and Bellatrix visit him there in Half-Blood Prince. Snape makes an Unbreakable Vow to Narcissa, with Bellatrix binding it.
In the books
Harry Potter and the Half-Blood PrinceChapter: 2
Narcissa and Bellatrix visit Snape’s house, where he makes the Unbreakable Vow.