The three of them started to stay at that place. They searched the house and saw Lily's picture and letter. The initials R.A.B. was written inside the fake locket and it was now that they discovered that it meant Regulus Arcturus Black. Then they remembered that they already had the real locket so they ask Kreacher and he kept it but now it was stolen by Mundungus.
According to Kreacher's tale, RAB is a death eater. Voldemort needed an elf for the ritual to turn the locket in to a Horcrux. He chose Kreacher to be the elf and had him drink the potion. That time, RAB realized how wrong it was to be a death eater and how wrong it is to allow his elf, Kreacher to be a sacrifice to Voldemort. So... RAB sends Kreacher back. Then RAB went back to the cave with Kreacher and asked Kreacher to switch the locket with a fake one then leave. RAB died there.
After hearing this, Harry returned the fake locket to Kreacher.

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My Chapter Summary On Harry Potter and the Deathly Hallows
Fanfictioncredits and honor to Author J.K. Rowling who will always be my inspiration in writing.