Star Wars: 10 Things You Might Not Know About Emperor Palpatine
8. He Broke The 'Rule Of Two' By Training Darth Maul
The ancient Sith Lord known as Darth Bane, who lived approximately one thousand years before the Clone Wars, instituted the ‘Rule of Two’ to ensure the survival of the Sith after the events of the Jedi-Sith War, in which the Sith were almost totally annihilated by the Jedi. This allowed the Sith to survive and plot in secret, with one master always surviving to train one apprentice, who would eventually take their place as the master.
In Legends, however, it is known that Sidious and Darth Plaguies, his master, broke the Rule of Two when Sidious took on Darth Maul as his apprentice. While in Canon, it is simply known that Darth Plaguies was killed by Sidious at some point prior to the Phantom Menace, in Legends, the exact moment is known. Sidious killed Plaguies the night before he is elected as Chancellor of the Galactic Republic.
This is an event we see in the films, as it happens after Queen Amidala leaves for Naboo with Jedi Master Qui-Gon Jinn and his Padawan, Obi-Wan Kenobi. We can assume the while Darth Maul is on Naboo facing Jinn and Kenobi, Sidious is at that very moment assassinating his own master back on Coruscant.
With Plaguies, Sidious and Maul, there were three Sith. The reason this was permitted is due to Sidious training Maul as a Sith Assassin, rather than as someone who he intended to one day become the next Sith master.