For there, at his throat, a medallion was swinging as Dumbledore continued to stalk menacingly in their direction. It was embossed with the coat of arms of Godric Gryffindor. And Tom Riddle's stinking, acidic, putrid essence was oozing out of every atom of the golden disc. Harry could think of no more fearsome defender for such a precious object. It was a stroke of genius.