Cascading Body Chain Necklace  Front to back long by VQSTUDIO

That is not part of the dress. The model is wearing a backless dress with a 'back necklace'. It was originally sold on easy but It is no longer there so you can try something like this.

As seen in
  • backless embellished dress embroidered dress embroidered diamonds sheer winter formal dress couture dress couture designer jewels diamond necklace diamonds dress embellished embelished dress long necklace vintage silver decoration black dress backless dress body chain wedding dress prom dress formal dress low back dress backless prom dress