Bring the Heart of Earth from the Plane of Earth to Xanthor is a main quest in Might and Magic VIII: Day of the Destroyer.
Walkthrough[]
To find the Heart of Earth, go to the entrance to the Plane of Earth and go inside.
In the course of completing this quest, once you are in the Plane of Earth, you will encounter the following monsters: Lesser Earth Elementals, Earth Elementals, Greater Earth Elementals, Chargers, Juggernauts, Demolishers, and Boulders.
Once inside the Plane of Earth, you will be in a little alcove which has the portal in it. Just outside the portal room you should see a corridor that branches to the right and left. Turn right and follow it along. You will go around a corner and up a hill. Keep going until you come to a large room with houses. You must cross this room and follow the corridor as it winds downhill, until you see a very large room on the Wizard Eye map. That room is your destination. When you get there, go on in. Cross the room to the far side and step into the elevator (the stone structure).
The elevator will automatically start down when you step onto it. Don't turn around, just wait. After a little, you will see the lower level. In front of you is a Grand Staircase. You can take either flight of stairs because they meet halfway up. At the top, you can also go either way - both doorways lead to the same place.
About halfway down the corridor (either corridor) you'll see a door. Pass it up for now and go on down and around the next corner. Ahead of you, in the middle of the wall, is a doorway (it's open; there's no door here). The Wizard Eye map should show it leading to a round room. This room has the Heart of Earth in it.
You will receive the experience for finding the Heart of Earth as soon as you leave the Plane of Earth (unless it is the last Heart you find, in which case you will receive the balance of the experience and fame when you report back to Xanthor).