Egeria was a Goa'uld queen.
She felt that it was wrong to use other species as slaves and forcefully take them as hosts, so she spawned a legion of prim'ta that lacked the domineering nature of the Goa'uld, becoming the queen mother of the Tok'ra.
Ra, the Supreme System Lord of the Goa'uld Empire, learned of Egeria's actions and captured her. He removed her from her host, and imprisoned her in a stasis jar on Pangar. She was discovered by the Pangarans buried in a tomb. They then found that they could use her Goa'uld off-spring to create a drug called Tretonin. Unfortunately, Tretonin harms the user's natural immune system. (SG1: "Crossroads", "Cure")
Egeria was the symbiote that resided in the alternate Teal'c who went back in time to Ancient Egypt in "Moebius, Part 2", with the temporally-displaced SG-1 travelling off-world to help Egeria take a new host, Egeria briefly serving as a symbiote to a woman Teal'c had fallen in love with to heal her injuries before moving on to create the Tok'ra. (SG1: "Moebius Squared")
When SG-1 arrived and discovered the drug and process to create it, they were concerned and contacted the Tok'ra. At first the Tok'ra had no moral qualms about humans using symbiotes for their purposes as they thought Goa'uld deserved to be treated no better than they had treated their hosts and slaves, but their opinion quickly changed when they learned that this was no ordinary queen but Egeria, the mother of all Tok'ra symbiotes. Unfortunately, she died shortly after blending with the Tok'ra Kelmaa's host but not before helping find a way to get the Pangarans off Tretonin. (SG1: "Cure")