9. Quinlan Vos
The former-Jedi turned Bounty Hunter, Quinlan Vos fell to the Dark Side as a result of Asajj Ventress' exploitation of his anger, in order for the pair to assassinate Count Dooku.
The Jedi Master was commissioned to pair with Ventress, and the two conducted several weeks of training, during which they developed feelings for one another.
Following his fall to the Dark Side, Vos underwent Nightbrother training on Dathomir, under the instruction of Ventress; this gave the former-Jedi Master experience with both the Light and Dark Sides of the Force, giving him an advantage over his foes due to his ability to draw on both Jedi and Dathomirian training.
He was captured by Dooku, and manipulated into pledging himself as Dooku's latest apprentice, following him learning that Ventress had murdered his former Master, breaking is trust in Ventress.
Vos was now a fully-fledged agent of the Dark Side.
Being trained by the Jedi, Ventress and Dooku gave Vos a huge array of experience to draw upon in order to succeed in lightsaber and Force combat.