Introduced in Disney's Episode 8, Star Wars: The Last Jedi, the Executioner Troopers were also known as the First Order Stormtrooper Executioners. Their purpose was to execute all First Order soldiers guilty of treason, with their powerful laser axes. Considered at times a test to see whether the stormtrooper would make it with the Stormtrooper Corps.
These Troopers were trained to deal with anyone disloyal to the First order, they were not from a specialist unit, but were instead randomly picked among regular stormtroopers depending on various assignments, and their identity was left anonymous. They wear armour with black carbon-finish accents with their serial numbers never broadcasted to squadmate's helmet displays. Run into one of these guys and - may the force be with you.