One of the greatest, and arguably strongest, of the Sith in the Star Wars Universe is Darth Plagueis. Though he never made his debut physically in any of the films, his reach stretched far past his demise. Plagueis was in fact the master of Darth Sidious, aka Emperor Palpatine, and was referenced in a scene between the Emperor and Anakin Skywalker in Episode 3: Revenge of the Sith. This old master of the Sith had apparently become so powerful in his studies in the Dark Side that he had found a way to not only become the strongest force wielder in the universe, he also managed to conquer death. In the tale that Palpatine paints for Anakin, his master, Plagueis, extensively studies midichlorians and uses his newfound knowledge in order to protect his loved ones from death. It's quite a big carrot to dangle in front of Anakin's face at that point in his life for sure.
Plagueis, unfortunately, was another casualty of the loss of the "Star Wars: Expanded Universe". For those not in the know, the Expanded Universe was essentially the stories told through novels, comics, and other forms of media outside of the movies that delved into the future and the past of the Star Wars Universe. In the novel titled appropriately enough, "Darth Plagueis", we were given a more in-depth look into Plagueis' life and his relationship with Palpatine. Plagueis believed that even though the Sith lived by a "Rule of Two", he would somehow manage to escape his eventual betrayal from his apprentice Palpatine, but like every other Sith in existence, he was wrong. Much like the story of Palpatine and Skywalker, the duo find themselves in a war of politics and attacks from the Sith themselves. Eventually, Palpatine manages to kill his master and begin the road into creating his Galactic Empire.
How would it be possible for Plagueis to be in Episode 7 you may ask? Well, the Force Awakens has another character in it named Snoke who appears to have a rumored appearance similar to that of Plagueis. Actor Andy Serkis will be bringing him to the screen, and he'll be another villain in the grab bag of villains that already populate the film. The former master of the Sith may have been "killed" by Darth Sidious, though we're talking about a villain who had control over life and death. Who's to say he didn't fake his own death, continue to amass his power behind the scenes, and wait for the perfect time to return to the universe with a new apprentice in the form of Kylo Ren? This would make for an excellent tie to the original trilogy while also creating it's own unique villain that hasn't been seen before in the films. The new trilogy needs an "Emperor" and Plagueis would certainly fit the bill.
While the Extended Universe may be "gone", Plagueis is still considered canon thanks to the speech from Revenge of the Sith. We'll have a brand new universe shown to us on December 17th with Star Wars Episode 7: The Force Awakens.