Palpatine is the Sith Lord responsible for the deaths of the entire Jedi Order a few days prior to my birth. He is one of the most powerful Force users to have ever lived, and his very words are capable of corrupting even the most resolved.

Exar Kun is, or...was a fallen Jedi. He lived 5,000 years before my birth, and started a galactic war that nearly destroyed the Jedi. He was so powerful and strong with the Dark Side that his spirit managed to transcend death, living on as a powerful ghost guarding three ancient artifacts.