Above the shoulder a helmet glinted. The light tapped the helmet and sparkled it into a goblet. Below the helmet a black body like a domino and a pair o black boots hurled themselves in the direction of the electrified fence.
After searching the definition foe this word, I realize goblet is a particular cup or glass that you would normally see when using for formal occasions. In the scene, the narrator is describing the man of who he can be. With the description that is given (helmet, black boots) this person may be a solider. The narrator used imagery to show how the light reflected into the glass.
Search engine Google “goblet definition”.