An Executioner is an individual formally employed by a government to terminally dispatch individuals sentenced to death in a court of law. It's also a class in Dungeons & Dragons 4th Edition.
Introduced in "Heroes of Shadow", the Executioner is a Striker class drawing from the Martial and Shadow power sources. Whilst an Assassin class had been released for the AEDU System in Dragon Magazine #379, by the time WotC got around to do a Shadow related splatbook, Essentials had reared its ugly head, and so we got stuck with the Executioner instead. They are described as specialist assassins, using a mixture of martial skills, poison use and dabblings in conduct politically motivated murders that would "send a message".
...Yeah, you can see they were struggling to define this as a separate class to the original assassin, can't you?
Executioners choose at 1st level to belong to either the Red Scales guild, specialized in close combat, or the League of Whispers, specialized in ranged attacks, which determines their at-will powers. Like a true Essentials class, they gain different features as they level up, most of the higher level ones being upgraded versions of early-level class features. At the Heroic Tier, they have a small pool of powers that they can choose from to customize their character, but once they hit Paragon & Epic tier, their use of powers falls away and their class focuses entirely on features gained from either their built-in paragon levels or their epic destiny, respectively.