The Cave of Trophonius, also known as "The Cave of Nightmares," was a deep chasm home to the oracle, Trophonius. In order to enter the cave to the oracle, one must first drink from the two fountains of Memory and Forgetfulness. Along with the spirit of Trophonius himself, the cave also contains dark ghostly bees, his sacred animal, and Water moccasins because snakes represent prophetic wisdom from deep in the earth.
Apollo and Meg McCaffrey find an entrance to the cave after being guided by the Arrow of Dodona. But the entrance is guarded by Yales, beasts who cannot be killed because they are endangered, so Meg decides to bind them with vines instead. Apollo drinks from the small black Mnemosyne and white Lethe steams and begins to act as if he's drunk. Meg skips the fountains to escape the Yales and acting like Apollo.
Once inside the cave, the two make their way to an island on a lake. The domed ceiling bristled with stalactites like black chandeliers. The water and island is infested with snakes but Apollo jumps in. Meg tries to make Apollo sing to calm them down but he doesn't listen so she does it to save him. It works and the snakes leave, but because of that Meg has offered herself to be the petitioner for the prophecy, she was unaware that he was going to the island to be the petitioner. Images of her life with her father and Nero began to be shown, that was what she was singing about. When Apollo demanded this to stop, Trophonius and ghostly bees appeared. Apollo begged his son to take him instead but Trophonius says Meg has already volunteered, then he sends the bees to swarm the two. Apollo is forced to give mouth-to-mouth resuscitation to save Meg. Trophonius then informs Apollo that Commodus is attempting to destroy the cave to get rid of Apollo. Trophonius tells him to let him do it as it is too exposed, unable to be defended, and he no longer wants to work for Commodus.
When escaping the cave, the two encounter Nanette, Eduardo, and another Blemmyae. They are setting up a bomb that will go off in five seconds to destroy the cave. Apollo allows it but convinces them to use it underwater saying there will be five more seconds, so they ended up destroying themselves as well while he and Meg escape with Peaches and his fellow Karpoi. The last prophecy of Trophonius was issued by Meg when she was placed on the Throne of Memory, it tells a Shakespearean sonnet prophecy regarding Camp Jupiter and the next oracle.