Heartstone

A Heartstone is an object which a phyrexian newt receives when it is created. The heartstone glows when the creature is living, and turns black when the creature dies. It is what gives the Phyrexians sentience. Every time a Phyrexian newt makes a mistake, the heart stone gets one scratch on its surface. Once the heartstone acquires a certain number of scratches, it will go black and the creature who has obtained its sentience from it will die.

Karn
After her sacrifice, Xantcha's heartstone was used by Urza to give sentience to the silver golem Karn. This particular heartstone had gone black, and was the storage for Karn's memory, soul and emotions. With more memories accumulating, Karn's heartstone expanded. Without knowing it, Karn also carried a trace of Phyrexian oil within him, which had been part of the Phyrexian heartstone from the start.