The Star Wars prequel trilogy finally introduced the Sith to viewers - and almost featured a third Sith. Darth Vader was referred to as the Dark Lord of the Sith n the original trilogy, but it wasn't until the prequels that George Lucas finally revealed what the Sith really were. There have been fewer Sith in Star Wars than Jedi, largely because the Sith are bound by the Rule of Two - which makes them much more menacing and mysterious.
Each of Lucas' prequel movies featured a different Sith apprentice. Star Wars: Episode I - The Phantom Menace focused on Darth Maul, Star Wars: Episode II - Attack of the Clones introduced Count Dooku, and Star Wars: Episode III - Revenge of the Sith finally transformed Anakin Skywalker into Darth Vader. But, surprisingly, Lucas intended the movies to have alluded to another prequel era Sith - Jedi Master Sifo-Dyas.
Lucas Intended Jedi Master Sifo-Dyas To Be A Third Prequel Sith
First name-dropped in Star Wars: Episode II - Attack of the Clones, Jedi Master Sifo-Dyas was the one who had commissioned the clone army. The movie never explained his motives, with subsequent Star Wars tie-ins explaining he was a Jedi who received visions of the Clone Wars from the Force and was killed in a Sith conspiracy. Lucas, however, originally had another explanation; according to Paul Duncan's The Star Wars Archives: Episode I - III 1999-2005, he intended Sifo-Dyas to be a secret Sith Lord.
Lucas' Sith Plans Fitted With His Interpretation Of The Rule Of Two
Some viewers would naturally have wondered how this fitted with the Rule of Two, given he would have been active at the same time as Count Dooku. But it's important to note this would have fitted with Lucas' interpretation of the Rule of Two, He imagined Sith to naturally be competitive in nature; there can actually be many Sith at any one time, but they will be unable to resist killing one another, until there were only ever two. The Rule of Two, Lucas believed, is not a doctrine so much as a default state that will always arise.
Lucas' logic may be consistent, but this would have simply turned Sifo-Dyas into yet another Palpatine lackey. The idea of another secret Sith would have undermined what made Maul, Dooku, and subsequently Darth Vader so compelling, making them too numerous. As it is, Star Wars has used the opportunity to make Sifo-Dyas a much more compelling character, with a rather more complex backstory.