"If our intent is always pure, the center will continue." -Black family motto
French: Si notre intention est toujours pure, le centre continuera
Latin: Pura mente nostra, si semper in centrum erit continue
Normal code: (1st to stop calls) Toujours Pur (2nd answers) Centrum Permanebit
Actual history lost until dug up by Regulus Black, after a House vote, Sirius ascend as Head
"Kreacher." The single, stiff word alerted the house elf to one of his masters needing to be attended. Sirius Black, hair as black as his name and scowl set, told the elf ominously
"This is the last order I'll ever give you, Kreacher. Whatever you do, protect Regulus." He paused and finished curtly
"If you have to Apparate away and back because he ordered you, do it. I'm not staying- but I might be able to get him clemency if Voldemort loses. Just keep him alive so I have the choice to try or send him away, alright?" Kreacher nodded silently, unsurprised when his master tightened his jaw and said quietly
"Bye, Kreacher."
Sirius' order and suggestion save Regulus' life less than two years after that conversation, and when Kreacher explains to a fuming Walburga, and Regulus wakes to explain what he discovered, Orion and Walburga pale.
It takes time- fifteen years, actually- but when Sirius comes to open the house to the Order, he arrives not to a dark, dank dungeon of a house. He comes home to his haggard parents and worried younger brother, who wrestles him into the small healers' suite in the subbasement. The environment itself is controlled by magic down there, and the formerly Slytherin-decorated house has been transformed to a stately home.
Remus is also wrestled into Regulus' healer-dungeon, as his parents grumblings call it. Harry is retrieved with all haste, when the Blacks rally together and demand it.
A very suspicious Regulus shuts his family, Remus and Harry into the Black library (relocated to his actually very expansive 'dungeon' before they permitted anyone else in the house). Once the Horcrux is removed, Harry is briefed on all of his actual options (such as fleeing the country and never returning). When he determines he'd rather fight, he is firmly ordered to do so in a Slytherin fashion.
So that's what he does. He even manages to turn the Malfoys' into spies, but that's another story.