From the introduction:
Grandfather Oshregaal was once a philosopher.
He dabbled in the black arcana of impossible spaces, mutable flesh, and mindless gods. He dug too deep. He learned too much. In the end, the creature named Oshregaal was very different from the creature that first picked up a forbidden tome.
To save what sanity he had left, he extracted some of the eldritch phantasmagoria from his head and stored it in a book. He buried the book, and was very surprised when a tree grew from it. This was the first black wind tree.