Starkiller - One of the most powerful Sith?
"The Apprentice is the photo negative of Luke Skywalker. He's been raised by Darth Vader, and is what Luke would have become if he had joined his father. Vader's not a very nice daddy. This guy has been raised to be a Jedi. When the Jedi use the Force, they respect it and don't overuse it. The bad guys - the Sith - keep testing their limits. Vader discovered this person who had the potential to be the most powerful Force user ever. He's up there with the top tier. He's extremely powerful. Vader has trained him in such a way that he just kept pushing his limitations, seeing how far he could use the Force. So, where a normal Jedi might use the Force to trick his way past a few stormtroopers, the apprentice might use the Force to bring down an adjacent building on top of those stormtroopers. He's extremely confident in everything he does. He's been trained by Vader to be an assassin, an unstoppable force."
―Haden Blackman
From the new star wars game Force Unleased, they obtained permission from George Lucus to create this character so i'm assuming it's canon.