From A Wiki of Ice and Fire
A greenseer is the name for the wise men of the children of the forest who had magical abilities that included power over nature and prophetic visions.
Greenseers had the greensight and were wargs as well. The First Men believed that they were responsible for carving the faces into weirwood trees and stories describe the greenseers seeing through the faces of their weirwoods, influencing animal and plant life. As the tales go, they used their power to shatter the Arm of Dorne into the island chain of the Stepstones.
The relationship between the greenseers of the children of the forest and the order of green men who protect the Isle of Faces in the God's Eye has not been specified. There is one greenseer left in the world - the Last Greenseer.
A Game of Thrones
In Winterfell Maester Luwin discusses the greensight and the greenseers with Bran. Bran remembers that Maester Luwin told him that the children of the forest had the greensight. Masters Luwin answers,
|“||“Some claimed to have that power. Their wise men were called greenseers. ||”|
When Bran asks, was it magic? Master Luwin replies,
|“||“Call it that for want of a better word, if you must. At heart it was only a different sort of knowledge. ||”|
When Bran asks him what was it? Master Luwin answers,
|“||“No one truly knows, Bran. The children are gone from the world, and their wisdom with them. It had to do with the faces in the trees, we think. The First Men believed that the greenseers could see through the eyes of the weirwoods. That was why they cut down the trees whenever they warred upon the children. Supposedly the greenseers also had the power over the beasts of the wood and the birds in the trees. Even fish.” ||”|
A Dance with Dragons
The Last Greenseer lives with the remaining children of the forest in a cave far north of the Wall. Bran Stark and his companions go in search of him, embarking on a perilous journey Beyond the Wall. They are led by Coldhands to the cave.