Karona
Dubbed a "False God". She's a manifestation of all Dominaria's magic, who rose from the fusion of the demonic queen Phage and the angelic crusader Akroma.
Despite her near-unlimited power, she was initially naive of her surroundings, running away from the sentinent beings who seem to be craving her awesome presence.
In an attempt to find her godly kin, Karona opened up five portals, one for each color of magic. While opening the Black portal, a dark realm was unveiled: the wreckage and remains of Phyrexia.
She was slain by two unlikely heroes, Ixidor's shadow servants, with Kamahl's Mirari Sword.
Upon her death, Karona was visited by Karn. He took her again to the plain of Argentum, where she was unmade and her true form, Jeska, returned into being. Karn revealed to her that she was a planeswalker. Karn left the living embodiement of the Mirari, Memnarch, in charge while he left with Jeska to teach her the ways of the planeswalkers, or something to that effect. They've yet to return.