Most people remember Harry Potter and the Order of the Phoenix as the book that introduced us to Dolores Umbridge, Thestrals, and a really angsty Harry Potter.
While all of those things are true, there's something in the fifth book that even the most dedicated of Harry Potter fans may have missed. And it deals with Horcruxes.
For those who don't know, a Horcrux is created when you place a part of your soul into an object after you've killed someone.
Now let's go back to book five. There's a passage that reads:
Did anything surprising stick out in that passage?
Also keep in mind that the locket that Harry and Dumbledore found had a letter enclosed.
Basically, the original locket/Horcrux was just sitting (in plain sight!) somewhere in Grimmauld Place because R.A.B., aka Regulus Arcturus Black, had taken it upon himself to try and expose the secret that ultimately led to Voldemort's demise.