Star Wars’ original trilogy is dominated by the presence of the Death Star and is predominately about the Rebel’s efforts to destroy Darth Sidious’ two planet-destroying superweapons. Years later, The Force Awakens introduced Starkiller Base, which was a derivative version of the Death Star. However, despite the status of the Death Star and Starkiller Base, there was another Sith weapon that was much more terrifying than either of them.
Darth Sidious used fear as a way to grow his power and control. In the Book of Sith, he explains, “Fear is the spark that drove my march to power. Even it fuels the engine of my Empire. The weak must be trained to fear the consequences of betrayal.” In that sense, Sidious used his weapons as symbols of the Empire’s military might and as a threat of destruction to anyone in the galaxy that might want to rebel against his rule.
Sidious was not the first Sith to use a superweapon to gain power, however. Darth Tanis was an ancient Sith who constructed a superweapon and housed it within a Sith temple on the Outer Rim world of Malachor. It, like the Death Stars, was powered by a giant kyber crystal. But while Tanis’ weapon never had the sheer destructive force as Sidious’ planet killers, it was much more terrifying and its activation inspired stories for millennia to come.
During the Battle of Malachor, the Sith superweapon was infamously used in battle. Jedi forces, after learning of the weapon’s kyber potential, attacked the Sith temple. While the details of the battle are still vague in canon, a Sith witch -- perhaps Tanis herself -- was said to have activated the weapon with a Holocron in defense of the Temple. However, something went terribly wrong, and it killed everyone in the battle by turning them into stone. There, they would stand -- Jedi and Sith alike -- petrified in eternal combat as a memorial of the temple’s power and a testament to the horrific destruction that it caused. This event became known as the Great Scourge of Malachor, and it would shape Sith teachings and Jedi legends for years to come.
Regardless of the many deaths, the battle was a success for the Sith because so many Jedi had fallen, and it became a way to teach Sith apprentices about the Dark Side's history. Before the events of The Phantom Menace, Darth Sidious took a young Maul to Malachor to educate him on the ancient Sith wars and inspire a hate for the Jedi in his young apprentice. The Sith Lord taught Maul that the Great Scourge of Malachor was a culmination of Jedi hypocrisy, and because of this, the Sith deserved to seek revenge. Sidious also made his apprentice inhale the ashes of dead warriors, forcing him to see terrifying visions of the horrific battle.
The Battle of Malachor also became a thing of Jedi legends. After Master Yoda instructs Ezra to visit the planet in Star Wars: Rebels Season 2, Episode 21, “Twilight of the Apprentice: Part 1,” Ashoka and Kanan are visibly worried. They tell Ezra that when they were younglings, Malachor was off-limits to Jedi. Regardless, Ashoka knows that there is always some truth in legends, and it’s not long after that they see what’s left of the Jedi and Sith warriors -- crumbling statues.