A rebel Goa'uld who hid on Earth for thousands of years, leading various cults throughout the world over the centuries. The Goa'uld Seth impersonated the ancient Egyptian god of chaos and embodiment of hostility and evil. He attempted to overthrow the Goa'uld System Lords, and had a price on his head from both the System Lords and the Tok'ra. Seth used Earth as a hiding place, and became marooned when the Stargate was buried in Giza, Egypt.
He continued in his lust for power and worship, forming several cults throughout the centuries. He impersonated the god Typhon in ancient Greece, to leading a suicide cult in eighteenth century England, to, hiding under the name Seth Fargough, leading a small group of followers north of Seattle at the end of the twentieth century -- where SG-1 and Jacob Carter found him.
Seth is the brother of Osiris. He banished Osiris and his queen, Isis, many thousands of years ago on Earth. He placed the two Goa'uld symbiotes into small stasis jars and, according to Egyptian mythology, cast them into the Nile River. Isis eventually perished, but Osiris escaped imprisonment, took a new host and fled Earth.
Seth - Jacob and SG-1 find Seth leading a cult of about 50 followers outside Seattle, where they confront and kill him.
The Curse - Osiris, once banished to oblivion by his brother Seth, is freed from his stasis jar and takes a new host -- Dr. Sarah Gardner.