A heart tree is a tree with a carved face that followers of the old gods pray, often within a godswood. It is often said that no man can tell a lie in front of a heart tree, as the old gods know when men are lying. Heart trees are almost always weirwoods, but other types of trees are used if weirwoods are not available. For instance, the heart tree of the Red Keep's godswood in King's Landing is a great oak.
—thoughts of Catelyn Stark
The heart tree had always frightened him; trees ought not have eyes, Bran thought, or leaves that looked like hands.—thoughts of Bran Stark
—septon to Lem