Number twelve, Grimmauld Place, London is a fictitious place in the Harry Potter books and is specifically mentioned in the fifth book.
"Grimmauld Place" is a pun on "grim old place". Template:Spoiler Template:HP Place The house was originally the ancestral home of Sirius Black's family. It eventually passed into the hands of Sirius, the "white sheep" and last remaining member of the traditionally very dark family line, and promptly fell into disrepair as its owner was locked up in Azkaban. The end result was a gloomy and unpleasant dwelling teeming with dust, decay and various malevolent enhancements, objects and pests. Due to the many enchantments the house appears small from outside, but inside is large and spacius. Inhabitants and "inhabitants" include a hysterical portrait of Sirius Black's mother who has a deep-seated hatred for non-Purebloods, an ancient and deeply mad house-elf named Kreacher who heartily seconds this opinion and a portrait of Phineas Nigellus who, before Dumbledore's death, was used for communication between The Order and Dumbledore.
Due to the high level of security measures implemented at Grimmauld Place, it was chosen as the headquarters of the reorganized Order of the Phoenix.
Harry inherits the house in Harry Potter and the Half-Blood Prince. Dumbledore has removed the Order of the Phoenix until he can be sure that Harry really owns it. The spells put in place by Sirius' family might have made sure that their home could only pass to family members or to pureblood wizards, in which case it would have belonged to Sirius' closest living relative: Bellatrix Lestrange. Dumbledore tested Harry's ownership of the house by having Harry give orders to Kreacher, the house-elf servant who is permanently tied to the house.
It is suspected that a locket found during the cleanup of the place in book 5 is one of Voldemort's Horcruxes, Slytherin's locket. If so, the locket may still be in the place, may have been thrown out with the rubbish, may have been stolen by Mundungus or may have been hidden by Kreacher.