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Medusa is a purported Craftmind, a Golden Age artificial intelligence designed for covert operations. She made contact with the Guardian under the guise of Eris Morn. Subsequent information suggests she may in fact be a manifestation of the Taken Vex Axis Mind, Quria, Blade Transform, although this is unconfirmed.
Both the Warmind Rasputin and the Golden Age AI Failsafe have stated that they have no record of Medusa's existence, despite Rasputin having been involved with virtually all military operations in the Sol System during the Golden Age, and Failsafe being affiliated with the Exodus Project. 
(Note: the following information is delivered by an unreliable narrator, and may not be accurate.)
Medusa was created during the Golden Age as a covert intelligence-gathering agent, and was assigned to observe the Project Amrita colony mission. She survived the passage through the singularity that consumed the Yang Liwei, and hid herself among the Awoken on the Distributary. She later followed Mara Sov as she left the Distributary and re-entered the Solar System, and eventually stationed herself as a covert observer within the Dreaming City.
List of appearances