Heart of Earth

The Heart of Earth is one of the five fragments of the great Blackrock Obelisk of Pagan. It is Lithos' object of power. It appears in  Ultima VIII.

Description
When the Obelisk was shattered by the Guardian near the conclusion of the great war, the Heart of Earth landed in Stone Cove, where Lithos found it and then stored it safely inside the Catacombs - specifically the Pit of Death part that housed the Conventicle of the Dead. Buried in his very earth and marked by a tombstone, the place became holy to him. It was there, where the Avatar found the Heart in the ancient grave, and later used it to drain Lithos of his powers, destroying him in the process. It then melted into the other fragments, reforming the Obelisk.

Trivia

 * Originally, removing the Heart would have triggered the undead invasion of Tenebrae, but the event got removed from the released game. See Plot Cuts in Ultima VIII.
 * The game can be broken by getting the heart even before meeting Lithos and then using it on him on the first meeting. Since he there appears in a cutscene, one has to drink a purple potion and attack one of the skeletons, so its starts to push the Avatar back towars where Lithos appears with its attacks. The Heart has to be double-clicked on to make the crosshair appear. One the Avatar is close, Lithos appears and can be absorbed, breaking the game.