Count Dooku

Count Dooku was a Jedi Knight who left the Order for unspecified reasons. Sometime between the Battle of Naboo and the Battle of Geonosis, he apprenticed himself to Darth Sideous and took the Sith name Darth Tyranus. He was killed by Anakin Skywalker at the Battle of Coruscant.

Trivia

His Sithian name of "Tyranus" is often misspelled as "Tyrannus."