Exile

You might be looking for Keepers of the Lost Cities Book Two: Exile.

Exile is where the elves keep their worst law breakers, and is for all of the intelligent species (except humans). Black crystals leap them to a desert called the Gateway to Exile. From there quicksand pulls them into a small cavern with a sand ceiling called the Entrance to Exile. In the entrance there is a slit in a wall that a magsidian key (pendant) fits into. When unlocked, the wall spins to reveal a doorway that leads to a narrow staircase. This is the beginning of Exile. Exile is in the center of the earth, dug out by dwarfs for elves to use. At the bottom of the staircase is another door that leading into a room call the Room Where Chances are Lost. Here a dwarf will meet the visitors and, if they are allowed to be there, the dwarf will be their guide. The magsidian key is inserted into this door and the dwarf guide inserts a magsidian coin to open the room. In the Room Where Chances are Lost cages line the ceiling and walls, the air smells bleak, and there is constant moaning from the prisoners. Above each of the cages there is a name plate that glows the name of the prisoner inside. The room is long and spiraled to allow for a maximum amount of cages.

Trips to Exile

 * During book 2: Exile, Sophie and Alden traveled to Exile to preform a mind break on Fintan. They traveled to Exile in the way described above and their dwarf guide was Krikor. Fintan managed to protect his secret, but still shattered, pulling Alden under with him. Alden shattered days later from guilt and Sophie did everything in her power to help, eventually bringing him out of the memory break. She almost died in the process because she was malfunctioning. While leaving from the mind break Sophie tried to read Prentice's mind without the success of healing him but learning new information and seeing Jolie in his dreams. Also during Book 4: Neverseen, when Sophie, Fitz, Keefe, Biana, Della, and The Collective break into Exile to steal Prentice and Fitz gets stabbed by an arthropleura.

Prisoners
Fintan (up to Everblaze)

Prentice (up to Neverseen)

Ogres