Cthaeh

The Cthae is a major antagonist in the book series The Kingkiller Chronicles, by Patrick Rothfuss. He may be the bigger bad of the whole series. He is a creature who lives in a great tree in the Fae Realm. He is immortal and omniscient  and can manipulate the future.

Early History
Not much is known about The Cthae. He is greatly feared by the Fae, who consider him the most evil being in existence. According to the Fae, he knows all possible futures, and will manipulate everyone he talks to into bringing about great tragedy and destruction, like sending forth a plague.

The Cthae is trapped in a great tree in the Fae Realm. A group of Fae called the Sithe are said to guard the tree and kill anyone who approaches so they can't carry out the Cthae's will. They will even kill those who the listener interacted with.

The Cthae is said to have influenced many disastrous historical events. Most notably, he spoke to Lanre to get him to start the Creation War and become Haliax. He also spoke to Iax/Jax to get him to steal the moon. (It is speculated by fans that Haliax and Iax are the same character.)

Speaking to Kvothe
When Kvothe was in the Fae realm with Felurian, he ran across The Cthae. It is unknown how he got past the Sithe. He spoke to Kvothe about his quest to find the Chandrian, and told Kvothe that the leader of the bandits he fought was in fact, Cinder. Afterwards, he tortured Kvothe bv telling him about how Denna was being physically abused by her patron. Kvothe ran away, but was mentally scarred by the encounter.

When Kvothe told this story to Chronicler and Bast, Bast was horrified, believing that Kvothe was acting under the influence of The Cthae. Later, Bast and Chronicler had an argument about whether The Cthae was actually capable of manipulating all events.