Describe the place Percy sees in his dream. Chapter 12

In his dream, Percy stood in a cavern. He was surrounded by misty creatures, who pulled at his cloak and compelled him to walk along the edge of the cavern. A voice wrapped itself around him, claiming that he'd been led astray and trying to make a deal with him. Then his mother appeared, tormented and frozen.... while the voice made demands. Then the dead whispered for him to wake.....

"The pit yawned so wide and was so completely black, I knew it must be bottomless. Yet I had a feeling that some-thing was trying to rise from the abyss, something huge and evil."