Ygg

"He took a mermaid to wife and stole fire from the Storm God and carved the first ship from a demon tree that fed on human flesh."

- Euron Greyjoy Ygg was a demonic tree conquered by the Grey King. It is only mentioned in Histories & Lore.

History
Ygg was a demonic tree that feasted on the flesh of humans. It was killed by the Grey King of the Iron Islands, and used its bark to build his first ship.

In the books
Ygg is a legendary demon tree who fed on human flesh. According to legend, the Grey King carved the first longship from Ygg's hard pale wood. It is unknown if Ygg was a weirwood, as the First Men of old hung the entrails of the condemned in the branches of heart trees.