In the cave is where things turn ugly. The mountain is hollow.  Inside is a sea of Boiling Oil and a city is floating on the mist above it. The only way to and from is Cheron, the boatman from hell. The kidnappers & prisoners are already across. (There is no way to know if this is real or a dream. 

 Once over to the city, The boys are captured and tortured (crucified at one point), at this time we meet Paulos. He is the son of Satan. He is opening the gate to hell. They get a slight tour in their descent to the ceremony. This is basically where Dante went. These worshiper are getting ready to open the gate and let the Devil out of his prison. (the city has a very strange feel to it, like part Greek, part Mesopotamia, part Egyptian). 

 Jake is forced to fight in an arena, a three headed dragon. He wins, but barely. Paulos is pleased to see him hill anything, claiming that he is actually winning the internal battle being waged.  Jake escapes and starts to let the victims out so they can all escape. There is no way across the river of oil. They decide the only way out is to climb to the top of the tallest building and climb out through the caves.  They evade the minions and make it to the top and escape, except Jake. The city sinks into the oil and the mountain appears to sink.  At school a few weeks later, Jake returns. Everyone thinks it's okay, but his eyes glow red and laughs with Paulos' voice.