Caorthannach

The Caorthannach is an ancient Demon sealed away within the Feywild after trying to destroy it in the Age of Anxiety.
When the Caorthannach is awake and at her full power, archfey are able to be killed. Her primary goal seems to be the destruction of the entire feywild.
Legends portray the Caorthannach as a great golden snake, longer than the Tree of All Seasons is tall, with eyes that contain the entire Feywild in miniature. Over the years since she was put to sleep by Telwyth-Teg, the roots of the tree have grown around her, making it much more difficult for her to slither out when she awakens.