Complex sentence: He escapes with Percy, Annabeth, and Rachel Elizabeth Dare, who have to go back into the Labyrinth to find Grover and Tyson, who had stayed behind to look for Pan, god of the Wild.
Split into simpler sentences: He escapes with Percy, Annabeth, and Rachel Elizabeth Dare, who have to go back into the Labyrinth to find Grover and Tyson. They had stayed behind to look for Pan, god of the Wild.