Darth Vax

Darth Vax was a female member of the Dark Council of the Sith Empire. Following her death, her son Darth Arrid took her seat on the Council, but Vax was unable to let go of her power and influence and began influencing him for her own agenda as a Force ghost. Her actions remained a secret from the rest of Dark Council for four years before they realized what was happening.

Behind the scenes
Darth Vax is mentioned in Star Wars: The Old Republic: Knights of the Fallen Empire in a codex entry obtained during Chapter 12: Visions in the Dark.