Darth Phobos is a long dead sith from the old days of the republic. She specialized in the use of force illusions and turning the fear of her opponents against them. She was enemies to both Jedi and The Sith alike, killing all who opposed her. During 2500 BBY, She established the Ancient Sith Cult, The Cult of Phobos, making it her own personal army against her enemies, sometimes even sending members on assassination assignments to both Jedi and Sith.
Her Terror was Stopped with a joint effort from both Jedi and Sith during the Raid of Hyperion. After a long battle, the Sith was killed,her remains left on Hyperion. After the Battle, what was left of the Cult of Phobos gathered her remains and placed them in a casket in a cave on Hyperion, where she would lay to rest.
However, before Her Death, Phobos cast her spirit through the force, losing her Physical Form. Long after her death, her spirit would look through the force, searching for an apprentice who gave in to their worse fears to assist her into finding a new body and return her to her former glory.
In the days of the Jedi Order towards the end of the republic and rise of the empire, Phobos was used during many trials for Padawans to achieve the rank of Jedi Knight. With her illusions during the Trial of Insight, only the brave and strong could pass the trial, defeating her vision.