Morgan Le Fay
A being who lived at the time of King Arthur and Camelot, described as a powerful sorceress. She was Arthur's half sister and adversary of Merlin. Like Merlin, she was a purported "magic" user, but at the time was an Ancient who was attempting to carry out her ascended instructions to thwart Merlin from developing his Anti-Ori weapon.
Thousands of years before, Morgan lived among the last of the Lanteans. Before she returned with her people through the Stargate after the first siege, she may have been a teacher. She designed a holographic program designed to teach young children about the Lantean System, and would later use her knowledge of it to her advantage.
Morgan and the other ascended beings saw the dangers to her kind by a weapon capable of destroying ascended beings, and when she eventually captured the Sangraal she hid it away on a world directly center to the alignment of planets Castiana, Sahal, and Vagonbrei.
For a time Morgan made her home on Vagonbrei, supposedly in a cave overlooking the village. Eventually King Arthur and his knights came to the world to look for her. The villagers, who revealed her location, believed Morgan cursed them with a sleep sickness. The Vagonbrei parasite was perhaps conjured by Morgan, though this is unlikely.
Thousands of years later Morgan, in ascended form, visited the city of Atlantis and masqueraded as an interactive learning hologram designed to teach young children about the Lantean System. She deceived Dr. Elizabeth Weir, but when Dr. Daniel Jackson and Vala Mal Doran arrived she gave them that which they were searching for -- the Stargate addresses for Castiana and Sahal.
In the thousands of years that had passed, Morgan had a change of heart. It was her hope that Jackson and SG-1 would eventually obtain enough information to find the Sangraal themselves. Daniel called her on her deception when he realized that she resembled pictorial references to Morgan in Vagonbrei's library. Morgan, discovered, confessed to her true form as an ascended being.
Initially Jackson and the others were unwilling to trust her, but Morgan explained that people and their beliefs can change. Daniel was well aware of the fine line she was walking by merely talking to unascended beings -- interference with lesser forms of life was prohibited by ascended -- but insisted on learning more beyond the two missing gate addresses.
When Vala asked that Morgan narrow the search by stating which planet where the weapon was hidden, Le Fay dematerialized -- pulled away by the others before she could answer. Vala, Daniel and Weir were left behind, wondering what punishment would befall her.
Morpheus - Daniel finds references to a powerful sorceress, Morgan Le Fay, in Vagonbrei's library books, as well as descriptions and / or drawings of her.
The Pegasus Project - Morgan reveals herself to Daniel, Vala and Weir, and shares the gate addresses to Castiana and Sahal before she is removed by the other Ancients.
Stargate: The Ark of Truth - Morgan helps SG-1 locate the Ark of Truth in the Ori galaxy, and reveals herself to Daniel. When Adria's power base is compromised by the Ark, Morgan attacks her, locking the two in eternal combat.