Jodah's Mirror was a powerful artifact in the possession of the archmage Jodah during Dominaria's Dark Age and Ice Age. During his apprenticeship, this magical mirror was given to him by the wandering mage Voska.
When Jodah later was led to Ith's prison by the Rag Man, he saw that Ith had been driven insane. After Ith went into a murderous rampage that resulted in the death of Mairsil and the destruction of the Conclave of Mages, Jodah managed to use the mirror to make him come to his senses.
Because the Fountain of Youth Jodah bathed in had dramatically slowed his aging, he remained practically unchanged through the following years while those around him died. The grief and despair that this caused made him descend into madness. To retain his sanity, he began to store his memories in the mirror. After wiping his memory clean, he restored it from the mirror, drained of its emotional attachment. He lived so long that he was granted the title of Archmage Eternal, and oversaw the City of Shadows as it changed into the School of the Unseen.
Creating the worldspell
Over 2000 years into the Ice Age that fell over Dominaria, Jodah was imprisoned by the necromancer Lim-Dûl, who muddled Jodah's brain with Fyndhorn Pollen, tricking him into believing he was a summoned creature and forced him to research magic for him. Jodah was saved by Jaya Ballard, a former apprentice who never got very far in the wizarding world. Jaya called upon the goddess Freyalise to cure him, placing them both in a position of debt towards the elven planeswalker.
In the aftermath of the battle Lim-Dûl's undead hordes, Jodah was summoned by Freyalise, who demanded to borrow the mirror as payment for his debt. After the mirror was used in the World Spell that ended the Ice Age, Freyalise returned it, but Jodah felt she had somehow altered it. From that moment on he did not dare to use it again.
Triggering Jaya's spark
Twenty years later, Jodah was again visited by Jaya, who now was possessed by Mairsil/Dûl. After she fatally stabbed him, Jodah used the mirror again. He was able to amplify his faltering magic with it and heal himself, but he also discovered what Freyalise had done to it. She had felt that someone near her had the planeswalkers' spark, and believed it to be Jodah. Annoyed by Jodah's irreverent attitude towards planeswalkers, she had enchanted the mirror to trigger the spark of anyone using it. But Jodah was certain that he did not possess the spark, and narrowly managed to escape the spell.
Faced with an immensely powerful Jaya/Dûl/Mairsil, Jodah realized that every time he had met Freyalise, someone else had been there: Jaya. After an intense duel, Jodah was in the right position and smashed his mirror in Jaya's face. The energy of Freyalise's spell was released, burning away the essence(s) of Mairsil and Lim-Dûl, and triggering her spark.