The Jedi and Sith have been on opposing sides of the Force for centuries in the Star Wars saga, but it seems that the Jedi themselves were responsible for creating the Sith in the first place. While both orders of the Force have been fighting each other since George Lucas' very first Star Wars movie hit theaters in 1977, the term Sith wasn't used on-screen until decades later, in Star Wars: Episode I - The Phantom Menace. However, the Sith themselves were always there.
Darth Vader was known as a Dark Lord of the Sith even before A New Hope released, and the Emperor later revealed to be his master. But what did all of that mean? Until the Star Wars prequel trilogy released in the late 1990s and early 2000s, the Sith's history in-canon was somewhat lacking, at least in comparison to the Jedi Order. That's partly why Lucas took the time to establish certain tenets of the Sith Order, such as Darth Bane's Rule of Two.
Still, despite the fact that eight movies and dozens of novels, comics, and video games have provided countless details about the Sith's history, there's still a grey area regarding their true origin. Interestingly, though, one scene in Star Wars: The Last Jedi may have confirmed a Star Wars Legends story about how the Sith were created, which would make their origin canon. And Luke Skywalker blames the Jedi for all of it.
The Sith In The Star Wars Saga
Over the years, it's been believed that people who harness power from the dark side of the Force are Sith, but that's not true. Those people - namely Kylo Ren and Supreme Leader Snoke, as well as the Inquisitors - are simply Darksiders. So far, only a handful of actual Sith have appeared on the big screen (with even more being mentioned in the comics and in animated TV shows): Darth Maul, Count Dooku, Darth Vader, and Darth Sidious. Everyone else is merely using the dark side of the Force.
The original Star Wars trilogy gave the world an entire galaxy to explore both on-screen and off, but it was the prequel trilogy that truly delved into the past - particularly with the Clone Wars, the Sith, and the Jedi Order. As Yoda recounts at Qui-Gon Jinn's funeral, there are never more than two Sith at a time. After the Sith fell approximately one thousand years before the events of the Star Wars movies, Darth Bane instituted the Rule of Two and sent the Sith into hiding until such time they were ready to strike back.
It wasn't until Darth Sidious (Palpatine) and Darth Maul that the Sith felt capable of destroying the Jedi. Throughout the prequel trilogy, audiences learned more about the Sith's history, such as the infamous story of Darth Plagueis the Wise. Of course, much more was revealed in the movies' supplemental material as well as various franchise media, but the core aspect of the Sith's story has been kept off-screen, especially their real origin, which has actually been revealed already; fans just might not have realized.
The Hundred-Year Darkness & The Rogue Jedi
A story that was once considered canon but then later relegated to Star Wars Legends was the tale of the Hundred-Year Darkness and the unidentified rogue Jedi that turned on the Jedi Order to study the dark side of the Force. Outside of canon, that rogue Jedi was known as Ajunta Pall, who separated from the Jedi approximately 7,000 years before the Galactic Civil War and fought the Order alongside a group of Dark Jedi, who went on to found the Sith Order on the planet of Korriban.
It's a wonderful story, but one that's no longer canon. Ajunta Pall's role has been reassigned to an unnamed rogue Jedi, who may not even be canon anymore since the original, official Star Wars databank that contained his dark deeds has been wiped away. The Hundred-Year Darkness itself wasn't mentioned again - that is, until 2015. While the vast majority of the old Expanded Universe was allocated to Star Wars Legends, parts of those classic stories have made their way back into the fold. The Hundred-Year Darkness is one of those tales.
After first being mentioned in detail in the comic book series Tales of the Jedi, the Hundred-Year Darkness was again mentioned - in-canon - in the 2015 comic, Star Wars #9, "Showdown on the Smuggler's Moon, Part II", in which a holocron containing the teachings of Master Phin-Law Wo says, "Once we were brothers in the Force, but from the Hundred-Year Darkness were born the Sith." It's clear here that the Sith came from that same period of time, but what's unclear is how it happened. Star Wars mythology is quite particular, as fans know, and this Jedi Master doesn't mention anything about a rogue Jedi causing the Sith's creation.
Luke Made The Sith's Origin Canon In The Last Jedi
Comparing Master Phin-Law Wo's statement to the original one from Sith Lord Marka Ragnos from Tales of the Jedi - "Once we were mighty Jedi of the Republic, brothers in the Force. But the Great Schism between the dark side and the light turned Jedi against Jedi." - it's obvious that the main difference is the mention of the Jedi. Ragnos calls out the fact that they were all once Jedi, and this is something that Luke Skywalker, too, subtly mentions in Star Wars: The Last Jedi, even going as far as to blame the Jedi for the Sith's rise.
Luke reveals this as part of his teachings to Rey on Ahch-To: "Lesson two: Now that they're extinct, the Jedi are romanticized - deified. But if you strip away the myth and look at their deeds, from the birth of the Sith to the fall of the Republic, the legacy of the Jedi is failure. Hypocrisy; hubris." Here, Luke considers the Sith's creation to be the failure of the Jedi; it's the first mention of the Jedi in direct association with the Sith's formulation in canon, especially with regards to the Star Wars movies.
At this point, Star Wars fans may never get to see Korriban or hear the stories of Naga Sadow and Exar Kun on the big screen, but looking through all the materials that Lucasfilm has put out over the past few years, the Sith's birth has already been laid out, and it seems that the official stance, at least where Luke is concerned, is that the Jedi were responsible. We may not hear about the unnamed rogue Jedi, whose only mention in-canon appears to be on an old version of the Star Wars website, but it's plain that these former brothers in the Force split thousands of years ago, and during a one-hundred year period, they formed the Sith Order.