The tortured tale of Darth Vader is one of the most compelling in Star Wars. His history of a child born into indentured servitude turned self-loathing galactic tyrant evokes a feeling of sorrow that is difficult to match. Yet, it is small potatoes compared to the Sith Lord Darth Zannah.
Zannah, like Vader, sought to be a Jedi, and fell to the Dark Side. Unlike him, however, she helped establish the Rule of Two and even overthrew her master to become Dark Lord of the Sith. Zannah suffered enough to set her on the path of the Dark Side as a child, on one planet, in the span of mere weeks. Zannah has yet to have a significant presence in canon, but should Disney choose to revive her story, it would overshadow Vader's.
Her first appearance came in Jedi Vs Sith (2001) by Darko Makan and Ramón F. Bachs. Before Darth Bane became her master, Zannah was a child named Rain who was conscripted by the Jedi to fight the Sith as a child soldier on the planet Ruusan. Vader's life as Anakin Skywalker was filled with hardship, but even he was not forced into an active warzone by the Jedi as a youngling. The real tragedy is that Rain did not seek war or glory, she was merely afraid of her family leaving her behind. But as one of Yoda's best Star Wars quotes goes, fear leads to anger, anger leads to hate, hate leads to suffering, and eventually the Dark Side. She was later told by a force-sensitive creature that she would not be the Jedi Knight she dreamed of and would fall to the Dark Side. She then attempted suicide, watched her companion get killed by Jedi, lashed out with the force, and killed said Jedi, and was then found and collected by Darth Bane. Ruusan sealed her path to the Dark Side. For anyone else, this would have been enough, but Zannah doubled down on her trauma and maimed her cousin as he tried to save her from Bane, killed the crew of a transport (one of them being another child), and flew herself to another planet to reunite with Bane.
While it's undeniable that Vader's arc throughout Star Wars is tragic, it is nothing compared to the trauma that Rain endured. What makes the whole thing even more remarkable is that all of that is just her origin story, and doesn't even account for everything she witnessed as a full-fledged Sith Lord. Anakin Skywalker's path to the Dark Side is certainly understandable, but it's still easy to see how someone else might have endured the same events and come out better for it. On the other hand, it's hard to imagine anyone not turning evil after what Rain went through.
It has been established that Darth Bane's Sith empire was overthrown by the Jedi, but Zannah's re-canonization has great potential with the new High Republic Era being explored. With Darth Bane having a new story in Star Wars canon as of Star Wars #20, it is hard to predict what kind of a story Zannah will have if expanded upon. If Disney is looking for a tragic Star Wars backstory to rival Darth Vader, then there are few better options than Darth Zannah.