This necklace is made to resemble the heart locket from Undertale that both Chara and Asriel had, although sadly it cannot open. There are two locket models, one that is pixel like to match the game's style and one that has more detail just to look nice. I would suggest sizing it up a little bit for more detail and to make it go on a chain a little easier, and you will need supports for the part the chain goes on.
The author marked this model as their own original creation.