Weirwood

From A Wiki of Ice and Fire

The weirwood is a species of deciduous tree found all over Westeros. Weirwoods have white bark with five-pointed, blood-red leaves and sap.

Associated with the religion worshiping the Old Gods, a single weirwood was planted in the godswood of all noble houses. Many southern houses cut down these trees when the Faith of the Seven gained favor, but they remain in the North. Groves of these trees can also be found above the Wall. It is known that the children of the forest carved faces into many weirwoods, but it not known why. Sap is allowed to dry in portions of these carved faces, resulting in disconcerting, blood-red eyes. Such trees are called heart trees and are considered sacred in this religion.