- "The Slytherin colours of emerald and silver were everywhere, draping the bed, the walls and the windows. The Black family crest was painstakingly painted over the bed, along with its motto Toujours Pur. Beneath this was a collection of yellow newspaper cuttings, all stuck together to make a ragged collage."
As 12 Grimmauld Place has been inhabited by the Black family for many generations, it is presumed that the room was used by another member of the Black family prior to Regulus's childhood, possibly as a bedroom.
After the death of Regulus, it is likely that the room was left untouched as a sign of respect for the deceased Death Eater. In 1997, the room was described as dusty and untouched, indicating that it has not been inhabited for many years.
Second Wizarding War
12 Grimmauld Place was used as a hideout by Harry Potter, Ron Weasley, and Hermione Granger in the same year. They were fulfilling the mission Albus Dumbledore had given them, to find and destroy all of Tom Riddle's Horcruxes, and were investigating the only lead they had, the initials R.A.B. which were hidden in a fake locket Horcrux.
While staying at the Black residence, Harry noticed that R.A.B. stood for Regulus Arcturus Black and searched his bedroom for the real locket Horcrux along with Ron and Hermione, however, they found nothing and soon learned that the locket had been stolen by Mundungus Fletcher.
After the war
It is unknown what happened to the room or the house after the fall of Voldemort and the climax of the War.
The room is described as slightly smaller than Sirius's. A sign is hung above the threshold which reads: Do Not Enter Without the Express Permission of Regulus Arcturus Black. The walls, windows and bed are draped with Slytherin's trademark colours: emerald and silver. The Black family crest is painted over the bed along with the family motto: Toujours Pur, which is French for 'Always Pure'.
An old Slytherin Quidditch team photograph adorns one of the walls, featuring Regulus, the Seeker, sitting in the middle of the a crowd of Slytherin students. When Regulus was a strong supporter of Voldemort, he collected newspaper cuttings about his idol and stuck them together to form a ragged collage.
Over time the room grew dusty and the newspaper cuttings turned yellow. When the room was searched by Snape, the drawers of a desk had been taken out and the contents had littered the floor. It is unknown if this mess was ever cleaned up.