The Locket

 is an armor item found in Asgore's Home, replacing the Heart Locket in the Genocide Route. Its offensive counterpart is the Real Knife. It is the Genocide Route armor item associated with the First Human.

Trivia

 * Despite being the toughest armor in the game, it never sees any real use as it's found right at the end of the game, and the final boss can only deal 1 HP of damage.
 * Considering the actions required to get the Locket, "You can feel it beating." may be a reference to "The Telltale Heart" by Edgar Allan Poe.