Darth Millennial is a character from the Star Wars Expanded Universe (also known as Star Wars Legends.) Though he does not physically appear in any fictional works, he is mentioned in several EU novels, including Darth Plagueis and The Glove of Darth Vader.
Darth Millennial was a three-eyed mutant Sith Lord who was apprentice to Darth Cognus. An heir to the lineage of Darth Bane who was tremendously powerful in the Dark Side of the Force, Millennial had the ability to see into the future.
Millennial often disagreed with his master, Darth Cognus, on many matters. This eventually lead him to become disillusioned with Darth Bane's Rule of Two, and later adopt his own philosophies of practicing the Dark Side, eventually founding his own religion, which could become the Prophets of the Dark Side. He proclaimed himself a chosen Prophet of the Force and would attract many followers. Those who did not believe in his new religion were labeled heretics and killed (which could be considered hypocrisy in its own right, as Darth Millennial himself was a heretic for breaking away from the mainstream Sith Order.)
Millennial's religion continued to exist for many years after his death, eventually being integrated into Emperor Palpatine's (also known as the Sith Lord Darth Sidious) Galactic Empire as the Empire's official religion, with Kadann being recognized as the Supreme Prophet of the Dark Side.