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.