Darth Zannah held the title of Dark Lord of the Sith, and served as the Sith apprentice to the esteemed Darth Bane. Following a prolonged war between the Sith and Jedi Orders, Zannah underwent training to perpetuate Bane's doctrines and the Rule of Two.
Roughly one thousand years before the Invasion of Naboo, the Sith Lord known as Darth Bane established the Rule of Two. This principle dictated that only two Sith could exist at any given time: a master embodying power, and an apprentice relentlessly pursuing it. In accordance with this Rule, Bane selected Darth Zannah as his apprentice, imparting he teachings to her.
Zannah managed to remain undetected by the Jedi Order. After Bane's demise, his apprentice continued the Rule of Two, which allowed the Sith to survive in secret while the Jedi mistakenly believed that the Sith had been eradicated. Despite the historical inaccuracies surrounding the identity of Bane's apprentice and the specifics of Bane's died, it is believed that he perished in a duel against Zannah on the Inner Rim planet of Ambria, a pattern that would be repeated by almost all Sith thereafter.
Darth Sidious' book, entitled The Secrets of the Sith, chronicled the history of the Sith Order, including its reformation under Darth Bane and his apprentice. Sidious lauded Zannah's master as a remarkable Sith Lord, acknowledging Bane as the originator of a Sith lineage that culminated in Sidious's own ascension to power as ruler of the Galactic Empire. Sidious also gave credit to Bane and his apprentice for initiating the Rule of Two, which ensured the Sith Order's continued existence through a series of Sith apprentices who ascended to the dark side's power by assassinating their masters, beginning with Bane's demise at the hands of his own student. Although not every Sith apprentice succeeded in overthrowing their masters, Sidious successfully concluded his apprenticeship by eliminating his master, Darth Plagueis.
Within canon, Darth Zannah received her initial, albeit implicit, mention in the "Sacrifice" episode, which served as the concluding installment of the sixth season of Star Wars: The Clone Wars. Her name was first explicitly revealed in the magazine Star Wars Helmet Collection 2. In Star Wars Legends continuity, Darth Bane's apprentice was initially referenced in Terry Brooks' novelization of Star Wars: Episode I The Phantom Menace. Due to the novelization's creation before Zannah's identity was established, it refers to Darth Bane's apprentice as "him." Brooks' novelization was informed by backstory notes provided by Star Wars creator George Lucas. The character of Zannah made her debut in the first issue of the comic series Star Wars: Jedi vs. Sith, released on April 18, 2001.