Ros Lai was the daughter of Nightsister Queen Zalem and an unknown Jedi Knight. When she was very young, her mother killed her father, beginning a resentment which would last the rest of her life. 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". All the Nightsisters on Dathomir believed that Ros Lai was crippled and therefore abused her. Even her mother would mock her. Later, however, she came out when she was watched by Vonya, Yani and Leela, as she transformed back into her true form. Ros Lai killed Leela when she asked for forgiveness, but she declared that none of them could ever forgive. Yani fled and left Vonya alone with Ros Lai. Ros wasn't strong enough to face Vonya who almost killed her in a fight. It was in this capacity that she discovered Quinlan Vos, who was posing as a slave to investigate a plot by Lai's mother to reawaken the ancient power of the Infinity Gate.
Ros Lai ran towards the star cruiser that belonged to Yag Shushin. She persuaded him to fly the ship to Quinlan to rescue him. Although the plan was successfully put into action, Quinlan was poisoned during his imprisonment with arterial worms. Therefore, Ros Lai infused him with some of her witch's blood due to having healing properties. Then they flew in the direction of the Infinity Gate, where Zalem was already using the super weapon to destroy the Galactic Republic's capital world of Coruscant. During their journey, they were discovered and shot by Vonya. Yag intercepted the shot that was supposed to kill Ros Lai and Quinlan Vos and died. Subsequently, their ship fired on Vonya and destroyed it, so they could go to Zalem to the stellar control station of the gate.
In order to gain Zalem's confidence, Lai stabbed Quinlan Vos with her fire blade, but in reality, it was just an illusion so that Quinlan was not killed. Instead, Lai was successful in killing Zalem, thus fulfilling her quest in avenging her father's death. With Zalem out of the picture, Ros Lai assumed the mantle as leader of the Nightsisters and ordered them to kill Quinlan for good. However, this triggered a mechanism and killed the Nightsisters, only leaving Ros Lai to be stunned. Quinlan reversed the weapon, destroying the Infinity Gate itself. 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