Taken from: Revenge of the Sith novelisation
This is Dooku, Darth Tyranus, Count of Serenno:Once a great Jedi Master, now an even greater Lord of the Sith, Dooku is a dark colossus bestriding the galaxy. Nemesis of the corrupt Republic, oriflamme of the principled Confederacy of Independent Systems, he is the very personification of shock and awe.
He was one of the most respected and powerful Jedi in the Order's twenty-five-thousand-year history, yet at the age of seventy Dooku's principles would no longer allow him to serve a Republic in which political power was for sale to the highest bidder. He'd said farewell to his former Padawan, Qui-Gon Jinn, now a legendary Master in his own right; he'd said farewell to his close friends on the Jedi Council, Mace Windu and the ancient Master Yoda; he'd said farewell to the Jedi Order itself.