Rule of Two

A long standing Sith tradition mandating that there could be only two sith, one master and one apprentice. This was started by Darth Bane after he eliminated all of his Sith rivals and thier apprentices. This tradition was carried on untill Darth Vader (the apprentice) sacrificed himself to save his son Luke Skywalker by throwing Emperor Palpatine, or Darth Sidious, (the master) therefore destroying the Sith Order. 
Various other darkside orders have existed without the Rule of Two.