An assassin is person who aims to kill a person without bringing to bear an attack with conventional weaponry. The use of assassins is well outside the rules of war but is, and was, a preferred way of removing political leaders under certain circumstances. A number of European kings were disposed of in this way.
The word assassin is derived from the name of a secret Muslim order that was mainly active in the 11th and 12th centuries. The Hassasin were regular users of hashish and the word actually means "hashish users". While other Muslims were the main target of their attacks, various crusaders fell victim to them in the 12th century.
The Far Isles Medieval Society has long been reputed to have an Assassin's Guild, but the official line is that the Guild "Does Not Exist" and that [ insert name here ] (frequently that of a lady, for some reason) "Is Not Its Head". The Clan of the Black Rat may or may not be members in good knocking-over (or knocking-off -- YMMV)