Kreacher returns to the sea cave with Regulus, who dies there

"M—Master Regulus took from his pocket a locket like the one the Dark Lord had. And he told Kreacher to take it and, when the basin was empty, to switch the lockets . . . And he ordered — Kreacher to leave — without him. And he told Kreacher—to go home—and never to tell my Mistress—what he had done—but to destroy— the first locket. And he drank—all the potion—and Kreacher swapped the lockets—and watched . . . as Master Regulus . . . was dragged beneath the water . . . and . . ."
-- Kreacher (DH10)

In 1979, Voldemort demanded the use of a house-elf, and Regulus sent Kreacher, a family elf that he was very fond of. However, the elf was left behind to die by Voldemort on the secret island in the sea cave where he’d hidden the locket Horcrux. Regulus had commanded Kreacher to come back, however, so the elf was compelled to return. The story Kreacher told was the last straw and Regulus became determined to destroy Voldemort.

From arrogant hints the Dark Lord had dropped, Regulus figured out what the locket was. Regulus ordered the elf to return to the island with him, substituted a fake horcrux for the real one, then told Kreacher to take the locket and destroy it. Inside the fake locket was the following note:

To the Dark Lord

I now I will be dead long before you read this but I want you to know that it was I who discovered your secret. I have stolen the real Horcrux and intend to destroy it as soon as I can.

I face death in the hope that when you meet your match you will be mortal once more.

R.A.B.

Regulus, who had drunk the potion in the stone basin to get to the locket, was attacked and drowned by the Inferi in the underground lake. He had forbidden Kreacher to tell “anyone in the family” what had happened, so they would be protected by not knowing (DH10).