The Heart of the land is an amulet made by Kikimi and her spirit animal, a Gila monster. They are one of the four heroes of the past, and are from Amaya. They found it in Heart of the Land book 5 of the second series.
This amulet gives the wielder the power to make sinkholes, tunnels, and make a big invisible shield. It can also make small earthquakes, and possibly make the wielder's bite venomous. The Heart gives power to anyone, even those who do not have a spirit animal.
It is a small stone on a chain with the shape of a Gila monster inside the stone.