Many years prior to 2000 BBY, the Sith Lord known as Darth Phobos met her end at the hands of a combined army of Jedi and Sith.
Both the Jedi and the Sith understood that Phobos and her followers presented a danger to them both. Therefore, they put aside their differences and allied against the cult, hoping to put an end to the Theelin Sith. The conflict concluded with a win for the Jedi/Sith alliance, and the defeat of Darth Phobos. Her cult lived on for a long time, but eventually died out around the time of the Fourth Great Schism in 2000 BBY.
Even though Phobos was believed to be dead, stories of her continued existence persisted for hundreds of years, turning into a myth within the Jedi Order.