Executioner's Robes is a piece of clothing that can only be obtained by using console commands. The only wearers of these clothes are Ahtar from Solitude and the executioner in Helgen during "Unbound."
The Executioner's Robes can only be obtained using the console command
player.additem 000CF8B3 X, X being the desired quantity. Though Ahtar wears these robes, killing him does not allow the Dragonborn to acquire the robes from his body.