The Mage is the physical form of MageConstellation. When The Serpent corrupted the Apex Stone, she was brought out into the world, where she was then split into five different aspects.
The Serpent managed to corrupt one of them, known as Lost One, while the two others managed to go into hiding. The rest went to Belkarth to seek the help of the Star-Gazers.
Once all the aspects are reunited, The Mage is able to regain her physical form, although still under the grasp of The Serpent. She then retreats to the Aetherian Archive, to prevent herself from causing any more harm.