Glistening oil, also known as Phyrexian oil, is a viral substance engineered by the Phyrexians to spread corruption wherever it travels. It is both a contagion and a means of colonization, used to compleat creatures into Phyrexians and power Phyrexian machines as well.
The infection of Karn
Without knowing it, Karn carried a trace of the Phyrexian oil within him, inside the Phyrexian heartstone granted to him long ago by Urza. Karn left traces of the oil in his travels from plane to plane, including the artificial world that he himself created: Mirrodin.
The infection of Mirrodin
The glistening oil planted a virus on Mirrodin that slowly spread. The first to be infected was Mirrodin's keeper, Memnarch. Unbeknownst to most Mirrans, Phyrexia was rebuilding itself, using their world as its host substrate. The metal structures and metal-infused inhabitants of Mirrodin were a perfect breeding ground for the spread of Phyrexian corruption. As the nascent civilization of Phyrexia expanded in secret, it struggled to evolve a unified purpose, and the danger for the plane of Mirrodin grew until it evolved into New Phyrexia.