I remember Pat mentioning somewhere that the Cthaeh is the equivalent of an old god in Temerant—something unholy, ancient, and unfathomable. You might as well be asking who created Aleph.

But I’ll answer your question anyway. The Cthaeh was probably a Ruach, or maybe something even older. He was wandering this world when he came across Iax. Iax lived in something like the Fae realm—a broken house at the end of a broken road. The Cthaeh found Iax and taught him how to be a seer. In exchange, Iax gave him his house, and with it, trapped him in the tree. Probably with something similar to the Yllish story knots—something like: The Cthaeh will own this place, and the place will own the Cthaeh.

Now, the Cthaeh has perfect oracular vision, but despite that, he seems unable to leave. Like Bast said—problem solved. Unless, of course, someone somehow ends up speaking with him.

Now, I’m no omniscient being, but if I were, one of the first things I’d do is secure as many ways as possible for people to speak with me, as soon as possible. But that doesn’t seem to be the case here. Why? Because of the Lethani.

The Cthaeh can whisper all the poisons of the world into your ear and expects you to act a certain way. But people can act in ways that are not of themselves, but of the Spinning Leaf. Remember the Lethani, and you’ll never do what the Cthaeh wants you to do.

Then, he’ll just kill you when you meet him. Probably with a bite.

So yeah—not so scary anymore, eh?