No single object - save maybe the good ship Weatherlight - has driven more Magic plots than has the Mirari. It was a wish-granting sphere forged by Karn, which he intended to use to gain understanding of Otaria, but which fell into terribly wrong hands during the events of Odyssey block.
Each of its users devastatingly underestimated it - the Aven zealot Kirtar went mad and slaughtered his friends, Cephalid overlord Aboshan died in an attempt to flood the whole world, the dementia mage Chainer accidentally released all the Nightmare monsters from his mind into the Cabal City. Only Kamahl was truly able to resist its malign influence, even as its power caused mass mutations during Onslaught block, and eventually used it to save his sister Jeska and turned it back over to Karn.
When Karn incorporated the artifact into the design of a guardian for his artificial plane Argentum (later to be named Mirrodin), it caused the resulting servitor, named Memnarch, to go mad with power, which precipitated the events of the first Mirrodin block. As of the end of that block's story, the Mirari had been separated from the dead Memnarch and placed into the hands of the people Karn charged with taking care of the plane. Well, we all know who runs Mirrodin now: The New Phyrexians. And if they got hold of the Mirari, that could be bad.
When mcbiskit asked Doug Beyer on his Tumblr blog, "Please please please tell me something about the whereabouts of the Mirari. Is it lost somewhere on New Phyrexia? It's one of the most powerful artifacts (flavorfully) ever created and remains a loose end!" the storyline honcho replied:
Karn used the Mirari to create Memnarch, the caretaker of Karn’s newly-created world of Argentum (later known as Mirrodin, later later known as New Phyrexia). When Memnarch went all power-mad and Glissa and co. defeated him, Memnarch, and presumably whatever still remained of the Mirari, fell into the planar core. Memnarch was destroyed.
We haven’t heard anything about the Mirari since then, and it didn’t play into the Scars of Mirrodin storyline (other than the core of the plane in general, corrupted by Karn’s exposure to Phyrexian oil, being the source of Phyrexian corruption that led to the rise of Phyrexia there). To my knowledge, we've said nothing specific about the fate of the Mirari itself — whether it perished along with Memnarch, or still exists in some form, deep in New Phyrexia’s core.
One cardinal rule of Magic story is "didn't die on screen, effectively isn't dead," and that applies to inanimate objects being intact or destroyed too. Having once designed a Phyrexianized version of the Mirari, I would be interested to see what that kind of uncontrollable power would look like in Phyrexian hands.