Yansu Grjak was a Nightsister hailing from the planet Dathomir and active during the era known as the Clone Wars, a significant period in galactic history.
During the decline of the Galactic Republic, Yansu served as a Nightsister shaman. Following disagreements in leadership within the Nightsisters and their subsequent departure from their native planet Dathomir, Yansu, together with her fellow sister Sai Sircu, commanded an army of Nightsisters who wielded lightsabers at the onset of the war between the Republic and the Confederacy of Independent Systems. Seeking the destruction of the Jedi Order, they allied themselves with Count Dooku.
During one engagement in the Clone Wars, Yansu was confronted by two Jedi. She dispatched her Nightsister companions to engage the Jedi in combat, while she made her escape aboard a Sith Infiltrator starship.
Later, she was dispatched to Rodia, where she was pursued by a pair of Jedi. Despite her powers, the Jedi used the Force to hurl crystals at her, weakening her, and ultimately leading to her death at their hands.