A willow tree in the heart of the Old Forest by the Withywindle. He was full of malice and hatred for walking things. His heart was rotten, but his strength and cunning were good, he was a master of winds, and his song and thought ran through the woods on both sides of the river. His spirit had drawn power out of the earth, and it had spread till it had nearly all the trees of the Old Forest under its dominion.

The hobbits encountered him in the forest, just before meeting Tom Bombadil.

He had much power in the Old Forest, although he was not as powerful as Tom Bombadil.
Encyclopedia entry originally written by Ereinion