The Egyptian god of chaos who embodied the principle of hostility – he was the adversary of the god Osiris – or of outright evil, even if his role was not altogether negative: Only he could withstand the stare of the Serpent of Chaos and only he had the weapons to which its flint scales were vulnerable. He was associated with foreign lands where Maat, the rule of justice, was unknown.

   During the second dynasty Seth became closely connected with Ash, the original god of the Upper Egyptian city of Ombos, whom he substituted as that city’s chief deity. [4] For a while during the third millennium BCE, Seth replaced Horus as the guardian of the pharaohs.
   As the story of Seth’s murder of Osiris and his eighty year war against Horus gained currency, Horus was restored to his preeminence [2]. During that war Seth tore out the left eye of his adversary but lost a foreleg and his testicles.

http://www.reshafim.org.il/ad/egypt/religion/osiris.htm