Aaaaah, cosplay. To truly nail it, you have to have all the bits and pieces. It's the finest of details that really make it -- like the kind of jewelry your characters wear. In this case, we're talking about a locket based on Asriel's mid-form.
To make the necklace comfortable to wear, Etsy creator Sunarry weighted the heart to prevent shifting and made the beads lighter -- an excellent choice for someone planning to stay in costume for any length of time.
For those without the inclination or the jewelry making skills, this piece is an excellent way to add the final touch to that Asriel cosplay.