Sifo-Dyas is a fictional character in the Star Wars universe, created for Star Wars Episode II: Attack of the Clones. He is only alluded to in the Star Wars prequel films, but appears as a character in Expanded Universe material.
Ten years before the events depicted in Star Wars the Clone Wars, Sifo-Dyas secretly commissioned (by secret demand from Senator Palpatine) a clone army on Kamino, ostensibly to defend the Republic; at the same time, Palpatine requested that he delete records of the existence of Kamino from the Jedi library archive. Prior to this, Dooku had left the Jedi Order and had fallen in league with Darth Sidious. The Sith Lord informed Dooku of Sifo-Dyas' actions. At Sidious' behest and a final test of his allegiance, for Dooku to murder Sifo-Dyas. Sidious intended to use Sifo-Dyas only as a dead-end cover for the order of a clone army.
Dooku kept his old friend's body frozen for years until the InterGalactic Banking Clan initiated their plan to turn the Kaleesh warlord, General Grievous, into a great cyborg supreme strategist for the Separatist droid armies. Dooku used Sifo-Dyas for a blood transfusion for the General. Once Grievous' transformation into a cyborg had been successfully completed, Dooku gave him the very special gift of Master Sifo-Dyas' own blue-bladed lightsaber.
In Star Wars Episode II: Attack of the Clones, Obi-Wan Kenobi traveled to Kamino to investigate the source of the dart that silenced Padmé's attempted killer Zam Wesell, the Kaminoan prime minister, Lama Su, informed Obi-Wan that Sifo-Dyas had ordered the creation of a clone army on Kamino. The Jedi Council was not aware of the order until Kenobi informed them of it after he met with the Kaminoans. Jango Fett, a bounty hunter and father of Boba Fett, told Obi-Wan that he never heard of the name Sifo-Dyas. He also said that he was recruited by a man called Count Dooku/Tyranus on one of the moons of Bogden.
By the climax of Star Wars Episode II: Attack of the Clones, Sifo-Dyas' army was revealed, as deployed during the Battle of Geonosis. In Star Wars Episode III: Revenge of the Sith, it was used as the tool of an act of betrayal to destroy the Jedi Knights, in accordance with Emperor Palpatine's murderous Order 66.
In addition, 'Sido-Dyas' was originally only a false identity for Sidious — there was no real Jedi Master by that name, with Obi-Wan, Yoda and Mace Windu claiming never to have heard of him.
Lucas at one point said he would reveal Sifo-Dyas' backstory in Revenge of the Sith; it is instead explained in the novel Labyrinth of Evil. The omission of explanation of such a major plot element within the films is panned as a major failure on the part of Lucas and his creative team. His name was changed to "Zaifo-Vias" in Brazil, in order to avoid puns on his name (as Fodias is a conjugation of foder, a Portuguese obscenity meaning to fuck).
In Star Wars: Visionaries (a non-canon work), Sifo-Dyas is shown to be a Caucasian man with a small, brown goatee. Also in Visionaries, it is suggested that Sifo-Dyas' blood is used in General Grievous' blood transfusion.