Quietly plotting her revenge, Ros used the Force to transform her appearance. Because of her disguise as a hideously deformed old hag, the other women of the clan called her "the Rancor". She was relegated to the lowly position of tending to the slaves. It was in this capacity that she discovered Quinlan Vos. Posing as a slave, Vos was investigating a plot by Lai's mother to reawaken the ancient power of the Infinity Gate. With Quinlan's help, she killed her mother in an attempt to take control of her clan and the Infinity Gate. Quinlan went to the gun, so that the Infinity Gate would be destroyed. He fled with Ros Lai in Yag's starship to Coruscant where she was taken into custody. His reason to have Ros Lai alive was that he felt that there was good in her.
During the Clone Wars, Ros Lai left the Jedi important information that helped the Galactic Republic in their fight against the Nightsister's alliance with Count Dooku, leader of the Confederacy of Independent Systems. She also intended to ensure that Ziro the Hutt made good on a debt he owed her. Thanks to Ros Lai's assistance, she received a reward from the Republic.
- The Official Star Wars Fact File 127 (LAI1–2, Ros Lai)
- Jedi vs. Sith: The Essential Guide to the Force
- The Complete Star Wars Encyclopedia
- Star Wars: The Clone Wars: Jedi Alliance: Prima Official Game Guide
- "Everything You Always Wanted to Know about the Nightsisters but Were Afraid to Ask"—Star Wars Insider 122