The Sith Order has long been a thorn in the side of the Star Wars galaxy. Through the course of millennia, the Order has grown in strength, resulting in the destruction of countless worlds through an unfettered quest for ultimate power and dominance. They have suffered just as many victories as they have defeats.
As Force users who rely on the dark side to wield destructive power, the Sith care little for the well-being of others. This ideology was fostered throughout the Order's history as it refined its teachings, mandates, and structure in order to accommodate its own chaotic ways. This allowed the Sith to conquer the galaxy several times.
The Hundred-Year Darkness
According to Legends canon, the Sith began during an ancient period of Jedi history known as the Hundred Years Darkness. It was during this period that Jedi Master Ajunta Pall began diving too deeply into the ways of the dark side, which corrupted his mind and turned him into the very first Sith acolyte.
This put him in direct conflict with the Jedi Order, who exiled him. He took a number of students with him who shared his ideology of passion over pacifism, and the Sith Order was created. This event was canonized in the Disney timeline without a reference to Pall, but it still remains one of the greatest unanswered questions about Sith history.
Establishing A Base On Korriban
Following exile, the Sith set up operations on a number of planets, including Dromund Kaas, Mustafar, and Tython. Their main planet, however, was known as Korriban, the ancient Sith world and a key facet of their mythology where they built temples and catacombs, including the Valley of the Dark Lords. It was here that a vast sum of knowledge and dark side power was amassed.
Later, this planet became known as Moraband, but the infamy remained intact. It became a springboard for attacks against the Old Republic and the Jedi Order, as well as a means to further dive into the teachings of the dark side, and refine their ideological view of the Force.
Conquering Coruscant
Most Star Wars fans are well aware of how Coruscant fell to Darth Sidious in the films, but it wasn't the first time the Sith managed to take the planet. 5,000 years before the Battle of Yavin depicted in the first Star Wars film, the Sith led a campaign against Coruscant and managed to seize it.
Later, the combined Republic/Jedi forces managed to liberate the planet, and the Jed Order built their iconic temple on top of a Sith shrine in order to contain and eliminate the dark side energy within. Unbeknownst to the Jedi, this was actually a victory for the Sith, since the dark side energy would slowly debilitate the Jedi Order over the course of the next few thousand years.
The Malachor Incident
The title Malachor is rife with negative associations. In the popular Star Wars video game Knights of the Old Republic II - The Sith Lords, the planet Malachor-V was the site of a devastating catastrophe that killed thousands of people and left an open wound in the fabric of the Force itself.
Another planet bearing the Malachor name was an ancient Sith world where the Order constructed a series of superweapons, including a massive temple powered by a large kyber crystal. Just like Malachor-V, this planet suffered its own catastrophic event when the weapon misfired, instantly petrifying everyone on the surface. This event would live in infamy in both Sith and Jedi lore.
The Great Jedi Purge
When the Mandalorians began expanding rapidly throughout the galaxy, it thrust them into a headfirst war with the Old Republic. Eventually, the Jedi Order was dragged into the conflict, and the results of that terrible war were devastating. Many Jedi fell to the dark side through either violence or nihilism, which proved disastrous.
Revan and Malek, two extremely popular Sith Lords from the Old Republic era, abandoned their Jedi ways and attacked the weakened Republic. Though Revan was turned away from the dark side and later defeated Malek, it paved the way for a great purge of the Jedi at the hands of the Sith Triumvirate. Both the Republic and the Order nearly fell into extinction as a result.
The Fall Of The Order
A little over one thousand years before the Battle of Yavin, the Sith engaged in another war with the Republic, on a far greater scale. By this time, constant infighting and internal power struggles had weakened the cohesion of the Sith Order, leading to its demise at the hands of the Jedi Order and Republic forces.
When the dust settled, the Sith Lord Darth Bane fled into hiding and re-established the Sith Order under his infamous Rule of Two. No longer would anyone be allowed to claim the title of Sith Lord on a whim. From that point on, the Order operated in secrecy with just one Master, and one apprentice at any given time. This was perhaps the most pivotal moment in the Order's history.
The Rise Of Darth Sidious
With a thousand years of relative peace and prosperity under its belt, the Jedi Order largely ignored the Sith, believing them to be extinct. Their worlds had either been isolated or cleansed of Sith history, but Darth Bane's restructured Order kept counting down the centuries, passing knowledge from Master to apprentice.
Eventually, Sheev Palpatine became an apprentice of Darth Plageuis, and the two hatched a scheme to seize control of the Republic and rule it from within. Sidious later turned on Plageuis and took an apprentice of his own, while winning a political race for the title of Supreme Chancellor. This would be the start of the new Sith ascension.
The Fall Of The Jedi Order
Operating in secrecy and in plain sight of the Jedi Order, Sidious relied on the dark energies radiating from the ancient Sith shrine underneath their temple to weaken their Force powers. As a result, he was able to fool the Jedi Order while building up the Republic's clone army to fight the fabricated Clone Wars.
The Jedi were spread throughout the various galactic systems, leaving them vulnerable to Order 66. Their Clone troopers turned on their Jedi compatriots and murdered them without hesitation, while the newly christened Darth Vader decimated those within the Jedi Temple, leaving only a handful alive to carry on the legacy. The Sith had won the day, and the Galactic Empire was born.
The Destruction Of The Sith & Fulfillment Of A Prophecy
Three decades after the Jedi Order had been destroyed, the Galactic Empire under Darth Sidious was busy tightening its stranglehold on the galaxy. Under the guise of Emperor Palpatine, Sidious dissolved the Old Republic Senate system and crafted the infamous Death Star, which destroyed Alderaan before being vanquished at the Battle of Yavin.
In the wake of this defeat, Luke Skywalker rose up to claim the mantle of Jedi Knight for the first time in decades. In his lust for power, Sidious attempted to turn Skywalker to the dark side, only to fail. When he tried to murder him, Darth Vader intervened on behalf of his son, killing Palpatine, and ending the Sith Order in one move. This solidified an ancient Jedi prophecy regarding the Chosen One, who would bring balance back to the Force.
The Aftermath & Second Defeat
Darth Sidious cheated death in both the canonical Disney timeline and within Legends material. In the former, his spirit possessed a cloned body being held together with life support mechanisms on the planet Exogol. It proved to be one of the most nonsensical plot arcs in the much-maligned Disney trilogy.
In the Legends timeline which predates Disney canon, Sidious also kept his spirit alive by possessing various clones. In this timeline, he once again tried to turn Skywalker to the dark side, while choosing one of Han and Leia's children as a vessel for his spirit, so he could live again. His essence was eventually destroyed after being absorbed by Jedi Knight Empatojayos Brand and the spirits of all the Jedi who had passed on before that time.