I—oh. [Regulus pauses, thrown for a moment by the question, horcruxes are all he's been thinking about since the moment Kreacher had said the word and Regulus had realized the awful depth of truth behind it. He forgets sometimes that it's a secretive branch of Dark magic, even from most Dark wizards. And Sirius has always gone out of his way to avoid any trace of darkness on him.

He's still shining with that now, almost glowing with his determination to do right. He thinks he's imagining the burn of the Mark on his arm, the sinuous movement, the way it twines through him inside. He imagines a lot of things.]

He's cut off a part of his soul and put it into an object. He can't die.

[Then he reaches up to his neck and tugs on a silver chain, and pulls a green and silver necklace out from beneath his shirt.]

If I don't kill this, he won't die. He doesn't know I have it. No one knows except Kreacher, no one.