Darth Sion is a human Sith Lord of the Sith Triumvirate in Star Wars: Knights of the Old Republic II: The Sith Lords (2004), alongside Kreia and Darth Nihilus. He was voiced by Louis Mellis.
Held together by hatred, literally
Sion's body was destroyed long ago and is now a mass of fractured, blackened tissue held in one piece purely by dark side will. He cannot be killed by damage because damage is his ordinary condition. Beat him and he stands up again; the game does this to you more than once, and the repetition is the argument.
The best written of the three
Where Nihilus is a force of nature and Kreia is a thesis, Sion is a person, and a legible one. He is in constant agony, he has been in agony for decades, and the only thing keeping him conscious is that he refuses to give whoever hurt him the satisfaction. That is a comprehensible motive, and it makes him the Triumvirate member you can argue with.
How he is beaten
Not with a lightsaber. The player defeats Sion by persuading him that his endurance is not strength — that a man who needs pain to stay alive is being kept by it rather than sustained by it. He chooses to stop. It is one of the very few boss encounters in a Star Wars game resolved entirely by argument, and it is the most memorable thing in the second half of the game.
Canon status
Star Wars Legends. Nothing in current canon has depicted the Triumvirate.