Zannah, known as "Rain" during her childhood, was a Human female from the planet Somov Rit. She was recruited to fight for the Jedi's Army of Light during the Ruusan campaign of the New Sith Wars but, after arriving on Ruusan, was separated from the group during a Sith attack and presumed dead. Unbeknownst to the Jedi, she had been saved and befriended by a member of the native Bouncer species called Laa. When the Brotherhood of Darkness later unleashed a planet-wide Force storm, Rain survived by instinctively enveloping herself and Laa with a bubble of Force energy. Not long afterward, the young girl watched as Laa was haplessly killed by Jedi scouts. Stricken with anger and grief, she unwittingly gave in to the dark side and murdered the Jedi. Her display of rage attracted the attention of the war's only surviving Dark Lord, Darth Bane, who decided to take her as his apprentice. Rain cast away the handle of her youth and instead embraced her birth name, becoming Darth Zannah.
Zannah's preliminary Force training took place on the planet Ambria and involved the study of ancient Sith magic. In her tutelage under Darth Bane, Zannah executed a variety of missions designed to incite anarchist movements throughout the Galactic Republic, including manipulating the terrorist Anti-Republic Liberation Front into a botched kidnapping attempt on former Supreme Chancellor Tarsus Valorum. From Hetton, the ARLF's leader, Zannah also recovered information regarding the creation of holocrons, a secret highly sought by her Master, who later killed Hetton after Zannah tricked him into an attack on the Dark Lord. When the parasites that comprised Bane's body armor began to cause him concern, Zannah was tasked with infiltrating the Jedi Temple archives on Coruscant to obtain information about the creatures. Her mission complete, Zannah was surprised to encounter her cousin Darovit, who confessed that he had exposed her presence to the Jedi. Zannah fled Coruscant with him and met Darth Bane on Tython, where the Sith prepared to battle the pursuant Jedi. Despite their numbers, all of the Jedi were slain, although Bane was grievously wounded and on the brink of dying. Zannah returned with her Master to Ambria and demanded on threat of death that the local healer save the Dark Lord, which he agreed to do only after Zannah revealed the Sith presence on Ambria to the Jedi. She betrayed and killed the healer upon completion of the procedure and also used her power to drive Darovit mad. The respondent Jedi assumed that he was the Sith Lord they sought and killed him, effectively convincing the Jedi of the Sith's final eradication. With no surviving witnesses, the existence of Darth Zannah and her Master remained safely anonymous.