Siamun Muran is a character in the Yu-Gi-Oh! manga. This is the manga version of Shimon Muran. Simon served as a High Priest to Pharaoh Akhenamkhanen and vizier to Pharaoh Atem. His name means "son of Amun".
Siamun was originally one of the Pharaoh Akhenamkhanen's guardians, and he was granted the Millennium Key to hold. His Ka (or his monster spirit) that he summoned was "Exodia the Forbidden One" and he swore to seal it and never use it again after he destroyed the force that was attacking the Egyptian capital. He apparently gave up his position of guardian to Shada, and to him he relinquished the Millennium Key.
When Atem reexperienced his past through the Memory World, he awoke shortly before the exorcism of Dark Spirit of Chaos, with little knowledge of his life prior to that point. Siamun told Atem off for sleeping and Atem mistakenly called him "Grandpa". Siamun mistook Atem's lack of knowledge, as a symptom of his tiredness and explained the nature of monster ka and some of the roles of the Millennium Items to him.
Siamun designed Atem's tomb, filling it with several traps and Shadow Games to stop thieves. He informed Atem that Akhenamkhanen's tomb had been a prototype for this one and that no thief, regardless of skill, would be able to get past the traps of a royal tomb. However, as he was speaking Thief King Bakura entered the palace, carrying treasures he had stolen from Akhenamkhanen's tomb and dragging Akhenamkhanen's mummy with a piece of rope. Atem then drove Bakura off by summoning "The God of Obelisk".
When Zorc Necrophades was summoned and begins to march on the city, Siamun takes up the Millennium Key and summons Exodia to combat Zorc. Although Exodia experiences some success, eliminating the Army of the Dead, Zorc overpowers Exodia because its power is based on Siamun's, while Zorc can draw power from the darkness. Therefore, Exodia is destroyed and Siamun is killed.