Sai Sircu was a member of the Nightsisters from the planet Dathomir during the era known as the Clone Wars.
During the conflict between the Galactic Republic and the Confederacy of Independent Systems, Sai Sircu, along with her fellow Nightsister Yansu Grjak, commanded a military force of Nightsisters. To achieve the downfall of the Jedi Order, they allied themselves with Count Dooku and the Separatists. She and her sister first launched an attack on the Sedawan. However, her true objective, which was revealed later on Dathomir, was the domination of the galaxy.
On Dathomir, her home planet, Sai Sircu engaged in combat with two Jedi Knights, but she was unable to stop them from rescuing Luminara Unduli, a valuable prisoner. After the Jedi freed Unduli, they pursued Sircu, who was then confronted by clone troopers under Cody's leadership. She used her dark powers to defeat the clones, but the Jedi proved too powerful, forcing her to retreat. Subsequently, the Republic established a military presence on Dathomir, capturing numerous Nightsisters.
However, Sircu managed to evade the clone forces and rejoined her allies on the superweapon known as Devastation. These allies included Count Dooku, his apprentice Asajj Ventress, and Darth Sidious, who communicated via holotransmitter in a rare appearance. Dooku and Sircu assured Sidious that their weapon's power exceeded all expectations, and Ventress proposed a demonstration. Sidious agreed, and Ventress set a course for Dathomir. However, Sircu, enraged by the sudden betrayal, unleashed dark energy on Dooku and Ventress, forcing them from the bridge and seizing control of the ship.
While Dooku contacted his Master, Ventress engaged two of the six Jedi who had arrived to disable the Devastation. Sircu inadvertently aided her Jedi opponents when she attacked Ventress, allowing them to escape. Later, Anakin Skywalker and Mace Windu confronted her in the ship's main weapon chamber as she attempted to absorb the power from gathered crystals. Success would have made her extremely powerful. However, the two Jedi defeated her, and she died as she lost control of her power, resulting in an explosion of energy that destroyed the superweapon and the ship before it could target Coruscant or other important planets. All six Jedi, along with Dooku and Ventress, managed to escape to safety on their ships before the destruction.