Glistening oil, also known as Phyrexian oil or Ichor, 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. 
In post-apocalyptic Dominaria, black-aligned knights and wizards are tainted by the lands where Yawgmoth and his minions were defeated. You can see evidence of this corruption in the black ichor that runs out of their eyes. 
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.
- Befoul (Urza's Saga)
- Blackcleave Cliffs
- Ebony Treefolk
- Ichor Explosion
- Ichor Rats
- Ichor Slick
- Ichor Wellspring
- Ichorclaw Myr
- Inexorable Tide
- Instill Infection
- Loxodon Convert
- Mycosynth Wellspring
- Phyrexian Hulk (New Phyrexia)
- Phyrexian Rebirth
- Rotted Hystrix
- Scourge Servant
- Spread the Sickness 
- Steady Progress
- Unnatural Predation 
- Doug Beyer. (November 03, 2010.) "Infectiously Phyrexian Art", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (August 02, 2007.) "Ichor Eyes and the Mask of Yawgmoth", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Doug Beyer. (September 08, 2010.) "Phyrexia and the Vaultlord", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Doug Beyer. (October 06, 2010.) "Spreading the Infection", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Doug Beyer. (March 16, 2011.) "Six Secrets Behind the Sets", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Monty Ashley. (February 17, 2011.) "Unnaturally Predated", Daily MTG, magicthegathering.com, Wizards of the Coast.