A brand-new Star Wars book reveals when Count Dooku first recruited Asajj Ventress, which has created a major issue for the Sith Rule of Two. Ventress was introduced in the canon Star Wars timeline in Star Wars: The Clone Wars as a Nightsister of Dathomir and Count Dooku's Force-sensitive assassin. Ventress is therefore one of several non-Sith dark side Force-users in Star Wars movies and TV shows. However, this Star Wars book suggests that wasn't Count Dooku's long-term plan.
In the new book Star Wars Encyclopedia: The Comprehensive Guide to the Star Wars Galaxy, written by Dan Brooks, Megan Crouse, Amy Richau, Amy Ratcliffe, Brandon Wainerdi, Dan Zehr, and Kelly Knox, additional information about Count Dooku and Asajj Ventress' history has major implications for Dooku's Star Wars story. The book explains:
"In the following decade, Dooku emerges as the leader of a Separatist movement that persuades thousands of star systems to split away from the Republic. On Rattatak, he also recruits Asajj Ventress to be his apprentice."
The language of this passage from Star Wars Encyclopedia reveals Dooku's true intentions: usurping Darth Sidious/Palpatine and becoming the Sith master.
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What Dooku's Choice Means For Star Wars
This Description Of Ventress Reveals Dooku's Secret Plans
Although it may seem to be a small difference, the fact that Ventress is described as Dooku's "apprentice" rather than his acolyte (or even simply his assassin) in Star Wars Encyclopedia is significant. Based on the Sith Rule of Two, there can only be two Sith in the master/apprentice dynamic at any given time: one master and one apprentice. As is the nature of the Sith, eventually, the apprentice will kill the master and take on their own apprentice.
This section of Star Wars Encyclopedia suggests that Count Dooku had already broken this rule. In fact, Star Wars Encyclopedia confirmed that Ventress' recruitment took place before The Clone Wars. Evidently, Dooku had been defying the Sith Rule of Two for quite some time, training an apprentice even as he was meant to be the sole Sith apprentice. This makes it even more interesting that Dooku went on to try and recruit Obi-Wan Kenobi in Star Wars: Episode II - Attack of the Clones. Dooku was clearly making moves to overthrow Palpatine early on.
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Dooku Was Playing A Losing Game
While it's in keeping with the Sith power structure that Dooku was planning to overthrow Palpatine as his Sith master, he was doomed to fail. Not only was Palpatine easily one of Star Wars' most powerful Sith, but he also had an extensive, meticulously crafted plan that even someone as wise as Dooku could not have undone. Considering none of the Jedi were able to perceive Palpatine's true intentions, it simply wasn't in the cards for Dooku to actually defeat Palpatine and become the Sith master.
It's nevertheless interesting that this was Dooku's intention and that, apparently, Ventress was Dooku's Sith apprentice. Ventress proved to be a formidable Force-user, even going toe-to-toe with Anakin Skywalker and Obi-Wan Kenobi in The Clone Wars. She would no doubt have become a powerful Sith, although that future was derailed because Palpatine was always one step (or several steps) ahead. This new Star Wars book still adds significantly to Count Dooku and Asajj Ventress' stories, as it reveals that Dooku was indeed training Ventress as his apprentice, breaking the Sith Rule of Two.
Source: Star Wars Encyclopedia: The Comprehensive Guide to the Star Wars Galaxy