In one of the most influential moments in the Star Wars prequels, Palpatine introduced Anakin to the Tragedy of Darth Plagueis the Wise, who has appeared in canon. Besides inspiring countless memes, telling the young Jedi about Plagueis' ability to ward off death all but ensured that he would fall to the dark side. At the time, though, Anakin didn't know that Plagueis wasn't a figure of distant memory and mysterious legend.
Of course, Palpatine was the apprentice who killed Plagueis. And while murdering one's master might seem excessive, it was actually common practice for the Sith. Even in ancient times, backstabbing and self-promotion would get celebrated, and in the millennium leading up to the prequels, killing one's Sith master was also an expected practice. So, here's why Sith made a habit of killing teachers and how Plagueis killed his master.
Sith Killed Their Masters Because Of The Rule of Two
Darth Bane lived a thousand years before Star Wars' prequel trilogy. He was a renowned military leader and a skilled lightsaber duelist, but his biggest contribution to galactic history was something called the Rule of Two. When the Jedi destroyed the Brotherhood of Darkness during the Jedi-Sith war, Bane was the sole survivor. So, he created the Rule of Two to ensure that the Sith could rebuild their strength in the shadows.
The Rule of Two stated that there would only be two Sith at a time. When an apprentice had outgrown his master, he would kill him, take the mantle of master and find an apprentice of his own. It was a way to ensure that the Sith Order was always moving forward and becoming more powerful. So, that's what Plagueis did to his master -- a powerful and intelligent figure called Darth Tenebrous.
Darth Tenebrous Was Plagueis' Sith Master
Darth Tenebrous was a Bith, a species renown for its intelligence and scientific and mathematic advancements. As such, he approached the dark side as a rationalist, believing that it was simply a potent tool. Tenebrous even thought that the combination of Bith technology and the dark side would culminate with the Sith destroying the Jedi. Obviously, that belief was a bit arrogant, but he was still a powerful Sith in his own right.
Tenebrous also had a fixation with determining and shaping the future. In fact, he manipulated a Munn banker into meeting his future wife, and the product of that union was Plagueis, who became Tenebrous' apprentice as an infant. On top of his manipulative abilities, Tenebrous was a very skilled duelist, and he could also use a variant of red Force lighting. The most interesting part about Tenebrous, however, was his alter-ego. He didn't live in the shadows as a recluse -- rather, he was an extraordinary starship designer named Rugess Nome. He even designed Darth Maul's Sith Infiltrator -- the Scimitar.
Darth Plagueis Didn't Plan To Kill Tenebrous
One day, Tenebrous and Plagueis were exploring a deposit of Cortosis ore on the planet of Bal'demnic. However, someone sabotaged one of their probe droids, causing a massive explosion of lethane gas. Tenebrous created a Force bubble, which contained the explosion, but he didn't see a secondary threat -- pieces of the ceiling collapsing above him.
When Plagueis saw that, he didn't try and save his master from the falling rocks -- he actually made them fall faster with the Force. The dark side told him it was time to kill Tenebrous and become the Sith master. So, he helped the debris come down, and one of the stalactites impaled his master. As his master was dying, he gloated about his superiority, broke his master's neck and watched the midi-chlorians exit his body. Tenebrous' legacy would endure, though. In The Rise of Skywalker, Darth Sidious housed his massive fleet on the Sith world of Exegol, where he named his 26th Legion the "Tenebrous Legion."