Heartstone

From MTG Salvation Wiki

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. Everytime a Phyrexian newt makes a mistake, the heart stone gets one stratch on its surface. After a certain of number of scratches are on the surface of the heartstone, the heartstone will go black and the creature who has obtained its sentience from it will die.

Xantcha's heartstone was used to give sentience to Karn.This particular heartstone has gone black, and is the storage for Karn's memory, soul and emotions. With more memories accumulating, Karn's heartstone expanded.