The Cthaeh is a creature who dwells within the branches of a great tree, located in the middle of a clearing in the Fae Realm. It is either trapped or forced to stay there by the guardians of the tree, The Sithe. It is known to converse with those who approach it, often displaying a sort of omniscience about the world and the life of the visitor.
According to Bast, the Cthaeh is poisonous, hateful and contagious. It can see all possible futures with perfect accuracy.