Though she was believed to have been dispatched to rule other planets in Ra's name, inciting loyalty in the people's god and quelling any rebellions that appeared, it is in fact thought that she spent most of Ra's rule on Earth itself where she was exiled for plotting against her master.
She was put into a canopic jar which fell into the hands of Napoleon Bonaparte, along with other artifacts such as an Ark bomb and was later acquired by Nazi Germany. After World War II, it ended up in the hands of a Nazi war criminal and eventually, after his death, his son.
Teal'c said that tales are still told of the fall of Sekhmet; she allied herself with Apophis against Ra and she was punished in a manner so protracted and painful that the stories still circulate two thousand years later. (SG1: "Resurrection", "The Barque of Heaven")
It is unknown what happened to the artifacts after this time, but sometime before 2004 rogue NID agents had managed to acquire the artifacts, including Sekhmet's canopic jar which had fallen into the hands of the Nazis and eventually Dr. Keffler, the son of a Nazi war criminal.
The scientists, attempting to gain the knowledge of the Goa'uld, spliced her DNA with that of a human ovum so that they could create a Goa'uld/Human hybrid which they assumed would have the genetic knowledge of Sekhmet; they were partially successful.
They called their new creation Anna and used nanite acceleration technology which had been recovered from Argos and stored in Area 51 to speed up her growth rate, producing a fully formed woman in only a few days.
Keffler and his scientists proceeded to torture Sekhmet whenever she arose as the dominant personality, and indeed Keffler himself even tortured Anna to garner as much information as he could from the Goa'uld. Although Sekhmet began to surface more regularly the longer the tortures went on, she never offered the scientists what they wanted; information of the Goa'uld. Sekhmet even attempted to blackmail the scientists and, in one attempt, managed to escape and murder 32 people who were in the compound she was held in, even activating the bomb which was inside the Ark bomb, before Anna regained control of the body.
SG-1 soon arrived on the scene along with Dr. Bill Lee and he, with the help of Teal'c, managed to defuse the Goa'uld bomb. Though the team attempted to help Anna gain complete control of her body, they did not manage and Sekhmet kept coming to the surface more and more. Sekhmet gained control one last time and set fire to the cell, escaping. Anna at some point regained control of her body and as just Anna killed Keffler saying the evil she saw in herself (Sekhmet) was nothing compared to the evil she saw in him. Anna, unwilling to accept her life being a servant of the Goa'uld which was indeed part of her, activated a fail-safe device which Keffler had designed and so released a deadly toxin into her body, killing both beings inhabiting it.
Sekhmet was known as the daughter of Ra where she was known by several names including the Great Huntress and the Lady of Flame. These same myths spoke of powerful weapons at her command that were known as the Seven Arrows of Sekhmet and her Jaffa Guard had lion helms. At some point, legend spoke that she defied her father with her followers claiming that she worked to protect her people from the evil Ra. Regardless, she was cast aside and banished for her actions which saw her being imprisoned in darkness within a canopic jar. Her Seven Arrows were separated between Ra, Apophis, Yu, Osiris, Ba'al, Anubis and Amun under a blood-oath that they never unite the weapons nor return them to their mistress.
Russian scientists recovered the canopic jar holding the symbiote where they attempted to study her only for Sekhmet to awaken and take the female scientist as a host. Afterwards, Sekhmet began operating in the galaxy once more where her followers spoke of her rise of power. It was believed that she had some part to play in manipulating the timeline to her advantage through the use of the Aten device. The effects of her actions saw Stargate Command being destroyed with an alternate version of Jack O'Neill arriving in the present only to die from radiation wounds. This led to SG-1 being dispatched to P3X-314 where Sekhmet had a meeting with Lord Amun as she attempted to recover the Seven Arrows. (SG1: "Unleashed Ep 1")
- Sekhmet on the Wikipedia