Life After Darth The problem Bane addressed when he bound the Sith to secrecy and their number to a pair was that only under such conditions could dark side ambition be checked. But Emperor Palpatine eventually broke the rule of secrecy and "bent" the rule of two by keeping a multitude of dark siders in reserve. Like his emperor, Vader also bent the rules, secretly training Sith disciples of his own so that he himself might usurp the title of Sith Master. Among them were Lumiya, an Imperial double-agent-turned cyborg, and Flint, a stormtrooper whose father was killed by General Grievous during the Clone Wars.
Vader had planned to pit these pupils against one another as past Dark Lords had done, taking the victor as his Shadow Hand. The Sith Lord died before that could happen, however. Flint, who had looked to Vader as a father, felt the loss deeply. He retreated to the planet Vjun where he mourned within Vader's Bast Castle. It wasn't long before he was found there by Lumiya. Though they'd never seen one another, without words they came to a mutual understanding. A towering statue of Vader looked on as Flint ignited his lightsaber and Lumiya her Sith lightwhip. The furious battle left both warriors battered, but in the end Flint's despair was no match for Lumiya's hatred, and the roles of master and the apprentice were determined.
(source: Evil Never Dies: The Sith Dynasties)
