Setesh (also called Set, Seti, Setekh, Sutekh, Sutech, Setech, Typhon, or simply Seth) was the brother of Osiris and Isis and a rebel Goa'uld. Setesh was depicted as a strange (and mythological) creature, and the masked guards based on its likeness became the subject of many Jaffa jokes.
Several thousand years ago, Setesh did not agree with his brother Osiris that the Tau'ri should be civilized, as he thought that it would lead to danger. As such, he concocted a scheme to dethrone Osiris and claim his brother's empire as his own. He somehow managed to convince Osiris and Isis to strike at Ra with him, assuring them that they would be able to usurp the Sun God from his throne. The three soon set their plan in motion but, at the last possible moment, Setesh betrayed them and revealed their plans to Ra; his plan all along. Isis and Osiris hid in Kheb; however, at some stage they left either by force or by choice, were captured by Ra and stripped from their hosts, and their symbiotes were placed in Stasis jars to be imprisoned for eternity.
It was sometime after this event that the Tau'ri rebelled against their Goa'uld oppressors, and so Setesh abandoned the Stasis jars in a vain attempt to reach the Stargate before it was buried by his former subjects. Unable to flee the planet, Setesh was forced to live in secret. (RPG: "Living Gods: Stargate System Lords")
In 1999, the Tok'ra collected records of every past and present System Lord, but their intelligence on Setesh ended after the burying of the Ancient Egyptian Stargate. Believing that he had been unable to escape Earth, Jacob Carter returned to seek SG-1's aid in locating him.
Dr. Daniel Jackson discovered that Setesh had spent most of his time on Earth forming various cults, among them one where he impersonated the god Typhon in ancient Greece and a suicide cult in England near Stonehenge during the eighteenth century where his worshipers had died after slitting their own throats but Setesh' body was never found. Daniel then tracked his most recent operation to a small group living just north of Seattle, and learning his latest human host was the coincidentally named Seth Fargough.
Despite being stranded on Earth, Setesh retained an impressive cache of Goa'uld technology. He had enough Zat'nik'tels and Goa'uld grenades to supply a small army, a personal Kara kesh, as well as a set of transportation rings. He also had a significant supply of Nish'ta, which he used to brainwash people into serving him. He also worried about the System Lords and the Tok'ra trying to hunt him down, despite having spent 5,000 years isolated from any contact with either group.
SG-1, with the assistance of the Bureau of Alcohol, Tobacco and Firearms, manage to infiltrate the Cult of Setesh and break his influence over his followers by shooting them with a Zat. Setesh attempted to blend in with his worshipers and escape through the underground tunnel, but Selmak sensed his presence and removed his hood. Setesh uses his Kara kesh on Jacob and throws him against the wall. Jacob gives his Kara kesh to Captain Samantha Carter who chases Setesh and uses a Kara kesh to crush his midsection, resulting in Setesh' death at long last. (SG1: "Seth")