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-Great Mystery solved-
Sifo-Dyas was a Jedi Master in the Jedi Order, though he was killed shortly after the time of the Battle of Naboo. The Kaminoan prime minister Lama Su informed Obi-Wan Kenobi that Sifo-Dyas had ordered the creation of a clone army on Kamino to help the Galactic Republic. The Jedi Council was not aware of the order until Kenobi informed them of it after he met with the Kaminoans.
According to the novel : Labyrinth of Evil, Sifo-Dyas was a friend of Jedi Master Dooku, who shared his belief that the Jedi Order was not what it once was and that it shamed itself by serving a corrupt Senate. Dooku, upon beginning his training as Sith Lord Darth Tyranus, was told by his master Darth Sidious that Sifo-Dyas had placed the order for the Clone Army in secret. Dooku was ordered to murder Sifo-Dyas and erase Kamino from the Jedi Archives before he left the Jedi Order for good.
The truth about the Clone Army is left ambiguous- it is possible that Sifo-Dyas indeed ordered it, but it is just as likely that Sidious ordered it and possibly used Sifo-Dyas as a scapegoat, in order to cover his tracks and rid himself of Sifo-Dyas, for reasons unknown. It is possible Sidious felt that Sifo-Dyas's friendship with Dooku could jeopardize his training as a Sith Lord. With Qui-Gon Jinn and Sifo-Dyas dead, Dooku was more likely to be loyal to his new Sith master.
Regardless of who really placed the order, it was Dooku who came up with the clone host, Jango Fett. When Obi-Wan Kenobi arrived on Kamino, the Kaminoans told him Sifo-Dyas had ordered the clones, but when Obi-Wan had asked Jango about Sifo-Dyas, Fett reacted suspiciously: in an alien language, Mandalorian, since he was Mandalorian stated in Episode II, he told his son Boba to close the bedroom door where Jango's armor was hidden, then proceeded to claim that he had never heard of Sifo-Dyas, insisting that he was hired by "a man called Tyranus". Tyranus, as is revealed later, is the Sith name of Count Dooku. How much Jango knew about the Sith plot will remain a mystery; he was killed shortly after the meeting.
Nevertheless, the truth of the matter remains unstated in the Star Wars series. What is clear is that the Sith were involved with the clone order, given the fact that the clones were trained to obey Order 66 without question.
Eventually revealed and used at the behest of the Galactic Republic, this army was later turned against the guardians of peace and justice as the Galactic Empire was created.
Sifo-Dyas was known as Sido-Dyas in early scripts for Attack of the Clones, where it was intended to be a cover name for Darth Sidious, as Obi-Wan, nor the Jedi Council recognized the name in earlier drafts.