There are no assassins as such. You simply pay a sum of money to a Non Player Character (NPC) and the attempt is made. The higher the rank of the victim, and the larger the size of his fellowship. the more difficult the attempt will be and the more likely it becomes that your identity will be discovered. Up to a point, you can overcome these obstacles by paying more for assassins. Note, however, that it's very difficult to assassinate a king and almost as difficult to get a Great Noble. You cannot assassinate a player who is with an army.