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That's odd, there is something in the core.
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One of the many planes of Magic: The Gathering, Mirrodin (originally Argentum) was created by the golem planeswalker Karn as his home after he departed Dominaria. He then promptly fucked off to explore the rest of the MTG setting because sitting on a big shiny rock all day is boring as fuck, and left it under the control of Memnarch. Who naturally went crazy because of Phyrexian oil that Karn tracked in, and transformed it into a semi-organic plane for the purpose of growing his own Planeswalker spark and/or procuring more sweet Phyrexian cocaine. Thankfully a plucky band of heroes overthrew Memnarch, creating Mirrodin's fifth moon in the process.

Unfortunately, none of them thought to bring a dipstick along to check the oil levels, so the Phyrexian oil continued its incubation even without Memnarch, and eventually birthed a new generation of Phyrexians from the core of Mirrodin, who proceeded to conquer the surface and turn it into New Phyrexia.

Settings of Magic: The Gathering
Pre-revisionist: First Magic Sets - First Urza Block - Arabian Nights
Legends - Homelands - Ice Age - Mirage
Weatherlight Saga: Portal Starter Sets - Second Urza Block
Tempest Block - Masques Block - Invasion Block
Post-Weatherlight: Otaria Block - Mirrodin - Kamigawa - Ravnica - Time Spiral
After the Mending: Lorwyn - Alara - Zendikar - New Phyrexia
Innistrad - Return to Ravnica - Theros - Tarkir - Eldraine - Ikoria
Two-Block Paradigm: Kaladesh - Amonkhet - Ixalan
Post Two-Block Paradigm: Eldraine - Ikoria - Kaldheim - Strixhaven
Never in a standard set: Fiora (Where the Conspiracy sets take place) - Kylem (Battlebond)