Asajj Ventress

Despite being a bald, skeleton-like freak with no pupils and an awkward skirt that would not be practical when lightsaber fighting, Asajj Ventress was personally picked by both Darth Sidious and Count Dooku to fight Anakin Skywalker during the Clone War. Despite the strange attire of their new assassin (which really didn't come in handy on a jungle world like Yavin IV) she was able to successfully lure Anakin away from the Battle of Muunilinst and duel him, only to be cast down into a pit to her apparent death. Ah well, nice try.

Despite her supposed demise, Asajj did in fact survive, and went on to lead several more CIS campaigns during the Clone War before defecting to a cult known as the nightsisters and fighting General Grievous. She was not killed but she was defeated, and apparently went into hiding after this, never to be seen again...