伏線

The locket nobody could open

Slytherin’s locket passes through a cave, through Regulus and Kreacher, through Grimmauld Place, and through Umbridge’s neck before Ron destroys it. The note signed R.A.B. is the middle of that trail, not the start.

仕込み

1ハリー・ポッターと不死鳥の騎士団 · Cleaning Grimmauld Place
The Order finds a heavy locket nobody can open and throws it onto a rubbish pile. It is one cursed object among many in the Black house. Kreacher later steals things from that pile.
2ハリー・ポッターと謎のプリンス · The cave, and a note in a fake locket
Dumbledore and Harry drain the basin and take a locket. It is a copy. Inside, a note signed R.A.B. says the real Horcrux has been stolen and will be destroyed. The initials are not explained in that book.

回収

ハリー・ポッターと死の秘宝 · Kreacher’s story, then the forest

R.A.B. is Regulus Arcturus Black. He made Kreacher take him to the cave, swapped the lockets, and died there. Mundungus later stole the real locket from Grimmauld Place and lost it to Umbridge. Harry steals it back, and Ron strikes it with the sword of Gryffindor.