The Oracle of Trophonius was one of the four Oracles associated with Apollo, and probably the most dangerous of the quartet. The petitioner entering the temple would drink from both the Fountain of Memory and the Fountain of Forgetfulness in order to prepare the mind.
Assuming he or she survived the process, they would proceed to the Cave of Trophonius and be forced to experience untold horrors that brought forth "nightmarish verse." Assembled properly, the nightmarish verse acted as a portend to the future – albeit in the form of a sonnet.
Once the petitioner exited the cave, if not driven insane (or dead), he or she would sit on the Throne of Memory and the verse would be recited subconsciously. The priest would write down what the petitioner had recited, which would be considered the prophecy issued by the Oracle. Without the Throne of Memory, the petitioner would be forever trapped within the horrors of the cave.
The Oracle of Trophonius is currently located in Bluespring Caverns, Bedford Indiana. While there, the spirit of Trophonius asks for a favor; he requests Apollo to destroy the cave, thus making the oracle inaccessible to anyone.
Trophonius mentions that the location is too exposed and that Commodus would keep sending men down. Grudgingly, Apollo agrees, which is why when he leaves the cave and encounters three Blemmyae, he makes no attempt to stop them from blowing it up.