This Dark Jedi was a pale skinned, bald headed female that fought on the side of the Separtists during the Clone Wars
One of the Jedi's deadliest adversaries during the Clone Wars was ironically a Dark Jedi that been trained by one of their own.
Asajj Ventress was a young orphan on the war-torn world of Rattatak when she met a young Jedi named Ky Narec who had been stranded on the red world. Ky recognized the young girls sensitivity to the Force and decided to take it upon himself to train her. However, this proved a noble but terrible mistake on Narec's part when he was later murdered by a Warlord. This event led Ventress to become bitter at not only the warlord that killed her mentor but also at the Jedi who she saw as being partly to blame for abandoning one of their own.
Later, during the time of the Clone Wars, Asajj would exact her vengeance on the Jedi by joining Count Dooku. During her time with the Separtists, Asajj came across both Obi-Wan Kenobi and Anakin Skywalker. The latter clashed lightsabers several times with her including one incident above the planet Xagobah when Anakin fought Asajj off after she attacked Boba Fett's ship Slave I.