Star Wars Makes The First Sith War Canon | Screen Rant
The first adult novel in Lucasfilm's The High Republic initiative, Light of the Jedi, confirms the Great Sith War in Star Wars canon - and the fact that the Jedi won. Over the years, Disney's Lucasfilm has been slowly bringing various parts of Legends back into Star Wars canon; part of that has been pulling elements from Knights of the Old Republic, and other parts have been incorporating characters, like Grand Admiral Thrawn, as well as plot points into major projects. One thing that has remained absent, however, is the Great Sith War.
In Star Wars Legends, Freedon Nadd resurrected the old Sith Empire and inspired generations of dark side Force users to wage war against the Jedi and the galaxy. His actions ultimately led to the Sith Lord Exar Kun campaigning against the Republic in 3996 BBY. Exar Kun and the Sith were so successful that they almost wiped out the Jedi, partially with the help of Mandalorians who allied themselves with the Sith. The Great Sith War finally ended after the Republic retaliated against the Sith's conquest, defeating their legions, the Mandalorians, and eventually, Exar Kun himself with a series of counteroffensives later that same year.
Up until 2021, the Great Sith War, aka the First Sith War, remained part of Legends, but Charles Soule's Light of the Jedi novel confirms it in Star Wars canon. When discussing the potential militarization of the Jedi, similar to what the Jedi Order did during the Clone Wars (which takes place centuries after when Light of the Jedi is set), Jedi Master Oppo Rancisis says, "But we have been a military force in the past. In fact, our predecessors waged and won the Great Sith Wars." What's interesting is that not only is the Great Sith War namedropped, but it's pluralized as well, which has even bigger implications.
Although Legends' version of the Great Sith War had a long-lasting, profound impact on the galaxy, it was merely one conflict among many that century, which have been labelled as the Old Sith Wars. Right after the Jedi Civil War ended, the Sith Civil War began, but one Dark Lord took the opportunity to unite a splinter faction and attack the Jedi, resulting in the First Jedi Purge, thus ending the nearly 50 years of fighting. It's unclear if Light of the Jedi canonizes all of those battles as well, or if the Great Sith War is the only conflict that has been brought into continuity. But considering the name is pluralized, it's possible the Great Sith Wars is being used as a replacement name for the Old Sith Wars. If that's the case, then there's a lot of history Star Wars can now explore in continuity.
And the thing is, Great Sith Wars is the latest ancient war to be canonized, following previous mentions of the Mandalorian-Jedi War and Mandalorian Civil Wars. By doing this, Lucasfilm can add sizable events from the old Expanded Universe into the official timeline without worrying about true assimilation yet; these are more large, general sweeps rather than intricate inclusions. For example, Darth Revan was made canon through Star Wars: The Rise of Skywalker, but that doesn't confirm that he was once a Jedi or that the Rakatans and their Star Forge existed. However, these inclusions leave the door open for further exploration later on down the line.