In Egyptian mythology, Sopdu (also rendered Septu or Sopedu) was originally the scorching heat of the summer sun. Sopdu's name, meaning "with Sopd", derives from this heat arriving shortly after the star Sirius has its heliacal rising, and thus being seen as coming "with Sopdet", Sopdet being the deification of Sirius ("Sopd" is the masculine form of "Sopdet"). Indeed, it was said that Sopdet gave birth to this heat, and so Sopdu was seen as her child. The Greeks made a similar conclusion; the Greek name "Sirius" essentially means "scorcher".

The effects of the scorching of the sun lead many ancient cultures to see it as war-like, and the Egyptians were no different in this respect, with Sopdu consequently being seen as a war god. Because heliacal rising occurs in the east, and the sun's heat begins there daily, Sopdu was referred to as "Lord of the East", and had his greatest cult centre at the easternmost nome of Lower Egypt, which was named Per-Sopdu, meaning "place of Sopdu". The combination of being a war-god, and being associated with the easternmost edge, lead to Sopdu being depicted as an Asiatic warrior, with a shemset girdle and long axe, and more generally being said to guard Egypt's borders. When the Egyptians conquered Sinai, he was also thought to guard the turquoise mines, which predominantly lay within Sinai.

His name is composed of the hieroglyph for "sharp", a pointed triangle, and the 3rd person plural suffix (a Quail); thus a literal translation of his name is "sharp ones". However, the triangle glyph was really a representation of a plant thorn, which the Egyptians referred to as a "tooth", and so his name could be seen as the plural of "tooth", i.e. "teeth". Consequently, war-gods also being associated with death, he was said, in the Pyramid Texts, to protect the teeth of the deceased.

By the Middle Kingdom, as a war-deity, he became strongly associated the pharaoh, which, together with his being god of the sky, lead to an association with Horus, the sky god, who was said to be the pharaoh's patron. Consequently, Sopdu started being depicted as wearing the two falcon feathers as a headdress, that represented Horus, who was seen as a falcon. He also started being identified as the standard bearer of Horus - gaining the glyph of a falcon on a standard in his name. By this time, the plural suffix of his name, previously a glyph of a quail, was shown with the hieratic abbreviation - a swirl, which often leads to misinterpretations of the standard depicting the quail, which appears to have otherwise vanished. Eventually, the association with Horus lead to his identity gradually merging to Horus, and in the New Kingdom, he was referred to as Har-Septu, an aspect of Horus rather than an individual.

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