When Albus Dumbledore agreed to take Harry to the cave in Harry Potter and the Half Blood Prince as Albus believed that the cave was a secret hiding place of Voldemort’s. Albus then had to drink a potion as this horcrux was protected.

Then after the basin was emptied of this potion:

There was a locket that turned out being a fake horcrux and as Harry found out later there was a note from someone called R.A.B.

Harry then discovers in Harry Potter and the Deathly Hallows that R.A.B. was Sirius Black’s younger brother called Regulas and he tells Ron and Hermione.

They then remember that Kreacher had a locket and realizing that it is the horcrux Harry summons him.

Kreacher then tells them that the locket was taken by Mundungus Fletcher, but Kreacher also tells him about what happen to him too.

As Harry wanted Kreacher to explain the full story and Kreacher then told them that Lord Voldemort had taken Kreacher to this same cave.

Though Kreacher wasn’t meant to have come back and only did so because Regulas had told Kreacher too.

Anyway, Regulas probably questioned Kreacher and somehow realized that Voldemort had made a horcrux and he gets Kreacher to take him to the cave that Voldemort had taken him too. Regulas must have done what Albus Dumbledore did in this cave and drank the potion to get the horcrux.

He then gives the horcrux to Kreacher instructing him to return home and I think Regulas might’ve told Kreacher to destroy it but Kreacher didn’t know how.

Also, to protect the rest of family Kreacher wasn’t allowed to tell them what happened.

Then Kreacher returned home with the horcrux, without Regulas and people just thought that Regulas had disappeared. Harry then gave Kreacher the locket belonging to Regulas and sends Kreacher to find Mundungus Fletcher.

When Kreacher returns with Mundungus he tells Harry that he doesn’t have it anymore. As Dolores Umbridge took it. Harry, Hermione, and Ron then had to make a plan as they went to The Ministry of Magic while wearing disguises.

As they needed to get this horcrux from Dolores Umbridge. Though she just thinks that this locket is a family heirloom.

It is said however that Regulas wasn’t aware of this prophecy to do with Harry and Lord Voldemort. However, he was horrified about what happened and also what could have happened to Kreacher in that cave.

So, Regulas planned what he did to destroy whatever plans Voldemort had.