Ros Lai was a Human hybrid of the female persuasion, existing as a member of the Nightsisters on the planet of Dathomir.
The Nightsister Queen Zalem bore Ros Lai with a Jedi Knight. Her mother murdered her father in Ros's early years, instigating a lifelong animosity. Secretly plotting vengeance, Ros employed the Force to alter her appearance, adopting an unattractive, aged visage. Consequently, her clanmates dubbed her "the Rancor," assuming her to be infirm, and subjected her to abuse, with her mother joining in the mockery.
However, her true form was revealed when observed by Vonya, Yani and Leela, as she reverted to it. After Leela begged for absolution, Ros Lai slew her, declaring that forgiveness was impossible. Yani ran away, leaving Vonya to face Ros Lai alone. Ros was too weak to fight Vonya, who nearly killed her. At that time, she encountered the Jedi Quinlan Vos, undercover as an enslaved individual to investigate Zalem's scheme to reactivate the ancient power of the Infinity Gate.
Ros Lai hurried toward the star cruiser in the possession of the Givin Yag Shushin. She convinced him to pilot the vessel to Quinlan's location for a rescue mission. The plan was executed successfully, but Quinlan suffered poisoning from arterial worms during his captivity. Thus, Ros Lai infused him with her witch's blood, known for its healing properties. Subsequently, they steered toward the Infinity Gate, where Zalem was already harnessing the super weapon to obliterate Coruscant, the capital of the Galactic Republic. During their journey, Vonya detected and attacked them. Yag shielded Ros Lai and Quinlan Vos from the fatal shot, sacrificing himself. Following this, they retaliated, destroying Vonya's ship before proceeding to Zalem's location at the gate's stellar control station.
Lai stabbed Quinlan Vos with her fire blade to gain Zalem's trust, but it was merely an illusion to spare Quinlan's life. Instead, Lai succeeded in assassinating Zalem, thereby achieving vengeance for her father's demise. With Zalem eliminated, Ros Lai seized leadership of the Nightsisters and commanded them to execute Quinlan permanently. However, this activated a mechanism that killed the Nightsisters, leaving Ros Lai stunned. Quinlan reversed the weapon, destroying the Infinity Gate itself. He escaped with Ros Lai aboard Yag's starship to Coruscant, where she was apprehended. He chose to keep Ros Lai alive, believing in the presence of good within her.
In 28 BBY, she independently published her memoirs, although her account was met with skepticism at the time.
During the Clone Wars, Ros Lai gave the Jedi critical intelligence that aided the Galactic Republic in their war against the Nightsister's coalition with Count Dooku, who was the leader of the Confederacy of Independent Systems. She also sought to ensure that Ziro the Hutt honored a debt owed to her. Ros Lai's assistance resulted in a reward from the Republic.