Post-Ruusan Sith

The term "Post-Ruusan Sith" refers to any surviving Sith Lords post-Ruusan. Many dark siders have claimed to be Sith, though they have historically been considered to be pretenders and killed by the Banite Sith, the most powerful organization of Sith survivors post-Ruusan.

The Order of the Sith Lords/Banite Sith
The Order of the Sith Lords, also known as Bane's Sith, Banite Sith or simply the Sith were an ancient organization founded roughly a thousand years before the Battle of Yavin by Darth Bane, the sole Sith survivor of the New Sith Wars.

The guiding principle of the Banite Sith was the Rule of Two, which stated that there would only be two Sith allowed to exist at a time: a master and an apprentice. The Rule demanded that after learning everything the master had to teach, the apprentice would attempt to kill the master and take an apprentice of their own. The Banite Sith lasted for a millennium, their plan succeeding in achieving galactic control and creating the Galactic Empire. Somewhere a few masters before Darth Plagueis, one of the masters faked his own death rather than be killed, training a new apprentice and splitting the Banite Sith into two separate lines, unaware of each other. This second line eventually discarded the Rule of Two, realizing the Galactic Empire to be the work of other Sith, and began training their own small order of Sith about 500 in size as of 18 ABY.

The Banite Sith and their empire were eventually brought to an end by Luke Skywalker aboard the Death Star II. The last master of the Banite line was Darth Vader.

Non-Banite Sith
Every now and then, some dark side user would declare themselves to be a Sith Lord, usually without the power and knowledge needed to back it up. Most of these so-called "Sith" were hunted down and killed by the Banite Sith for their lack of respect, others managed to slip under the radar. The Banites considered themselves the only true Sith and were greatly offended by those outside their order claiming to be a Sith.