It could easily be Palpatine or Anakin. Therefore i say it refers to the sith in general.
In the phantom menace the Jedi council says the sith have been exstinct for a millenium and they don't believe they could return without their knowledge.
And Yoda says "ah! hard to see the darkside is!".
Haters of the prequels would say the Phantom Menace is Jar Jar, when jar jar appears star wars was destroyed from a certain point of view,lol.
In Episode II the growing power of the darkside clouds the jedis judgement in the force. The secret creation of a clone army happens right under the councils noses. Palpatine Sidious gets elected as Chancellor at the End of episode 1 and if there right when they burn Qui Gon's body. When Yoda says " always two there are. no more, no less. a master and an apprentice." Then Mace says "but which was destroyed the master or the apprentice" then the camera zooms in on sidious face.
Everything from the trade federation dispute and embargo and capture of naboo and its queen, to the seperatist movement the creation of the clones.
The Jedi being made generals and leaders of soldiers, Chancellor Valorum being romoved through the lies and treachery of palpatine who used padme to gain his ends.
The entire prequels are sidious pulling the strings and making the jedi move like pawns to the slaughter in his deadly game of revenge of the sith, the sith waited over a thousand years since the time of darth bane to exact their revenge.
Having already seen the original trilogy I knew full well when seeing Episode 1 for the first time that Palpatine and the Master of Maul where one and the same. Even so Lucas tried to keep it a secret until episode III's reveal. In Episode II a plot point was dropped about Sido Dyas the sith lord pretending to be a jedi who ordered the clones. Instead they changed the name to syfo dyaas, and in the EU made him a jedi manipulated into ordering the clones and murdered by Dooku.
Palpatine the puppetmaster trys to make the future he sees through the darkside of the force come true. Therefore by engineering an outcome he can claim to have foreseen it, as he does in return of the jedi. But Luke does just what the emporer least expects. He expects everyone to be coruptible by power, that is how Vader was originally seduced to the dark side.
The Jedi betrayed themselves once they went to war according to the novelization of revenge of the sith, they had lost from the very beginning because Jedi are peackeepers and not soldiers.