He staggers and collapses, dimly conscious of Kreacher reaching for the locket. The elf's face seems more bruised than usual, though it's hard to tell at this distance.
Regulus hears rather than sees the replacement hit the bottom of the basin. As he tries to clamber to his feet, he feels a clammy grip on one arm. He reaches for his wand with the other, but it too is seized. There is nothing to be done but breathe, taking in all the air he can.
They pull him inexorably down. He does not begrudge the Inferi their duty, but rather pities them; in death they are still caught in the machinations of some greater power, but in death he will be defiant.
His body, craving air, breathes in. The water pacifies his throat and jolts the rest of him aware. There's a wave passing through the lake—Kreacher rowing away—and the potion has refilled. The cave glows green like a canopy of malevolent stars.
He breathes once more, and, like the dawn of the new day, all the stars go out.