Darth Phobos

Darth Phobos was a female Theelin Sith Lord who was born on the planet Korriban and lived thousands of years before the outbreak of the Clone Wars. Cunning and manipulative, Phobos slew many of her Sith brethren in a bid for power, and the rest turned against her and attempted to kill her. Phobos survived the attempt on her life and assembled a fanatical cult that worshiped her, using the cultists to strike against Sith and Jedi alike and earning the nickname "The Hidden Fear" among the Sith. Eventually, a joint force of Jedi and Sith killed Phobos, but her cult continued to live on until the Fourth Great Schism.

Tales of Phobos' survival became an urban legend, and her image was incorporated into the Jedi Trials as a simulation for the "Trial of Insight." In 3 BBY, that simulation battled Dark Lord of the Sith Darth Vader's secret apprentice, Starkiller, and was defeated.

Text and facts from the “Darth Phobos” article on Wookieepedia, licensed under CC BY-SA 3.0. Modified for this site.