Summary
- Darth Bane's Rule of Two allowed for other dark side users in the Galactic Empire, resulting in many Imperial dark side organizations.
- The most powerful Imperial dark side users include the Grand Inquisitor, Kharys, Jerec, Mara Jade, and Lumiya.
- Lumiya, the Dark Lady of the Sith, is the most powerful Imperial dark side user after Darth Vader and Emperor Palpatine. She bested Luke Skywalker in their first duel.
The Star Wars franchise’s non-movie material from canon and Legends reveals that Emperor Palpatine and Darth Vader are not the only dark side users in the Galactic Empire. Darth Bane’s post-Ruusan Sith Order is defined by its Rule of Two, which only allows two Dark Lords of the Sith at a time, but this did not prevent Palpatine and Vader from training Force-sensitives to use the dark side as Sith in all but name. In both continuities, there are many Imperial dark side users, but few compare to the power of Darth Vader and the Emperor.
In Legends – the Star Wars franchise’s original timeline – there were numerous Imperial dark side-using organizations, including the Inquisitors, the Dark Side Elite, the Emperor’s Hands, and various unofficial Sith apprentices. When the Star Wars franchise gained a new continuity in 2014, the Empire’s cadre of dark side users was reduced to only the Inquisitors. Of all Vader and Palpatine’s Imperial dark side underlings in canon and Legends, the following beings are the most powerful.
Your browser does not support the video tag. 9 The Grand Inquisitor 
The new Star Wars canon’s version of the Imperial Inquisitors is intentionally kept weak to prevent them from becoming threats to Darth Vader and Emperor Palpatine. Despite this, one Inquisitor’s martial skills and dark side powers are more impressive than his colleagues. The unnamed Pau'an and former Jedi Temple Guard became the Grand Inquisitor for both his power and skill at hunting down Jedi. The Grand Inquisitor is hardly a threat to Darth Vader, but he proved to be a deadly opponent to Kanan Jarrus and, as a dark side spirit, to Luke Skywalker.
8 Kharys 
While not an official Sith, the S'kytri known as Kharys was trained by Darth Vader himself and ruled her home planet on behalf of the Galactic Empire. Kharys was extremely skilled in lightsaber combat, taking advantage of her ability to fly and mastering the rare variant of lightsaber combat Form VII, Trispzest, a style exclusive to beings who can fly. Kharys also mastered obscure and nightmarish Sith powers, such as the conjuring of Smoke Demons. Kharys could be considered one of the few beings with the potential to become a true Sith, if Vader or Palpatine had died before she did.
7 Hethrir 
Similar to Kharys, Hethrir was trained by Darth Vader to use the dark side of the Force, but rather than being given rule over his home planet, he became the Procurator of Justice, putting him in charge of the Empire’s draconian justice system. Hethrir was skilled in lightsaber combat and his training made him a powerful dark side user, but he was arguably less formidable in a battle than Kharys. Hethrir’s true skills were his ability to suppress the Force-connection of others and his prowess as a leader. Years after Dark Empire, Hethrir created a sub-faction of Imperial Remnant known as the Empire Reborn.
