The Forgotten Cell can be found after playing Hide n' Seek with Mody in the Caverns. Once you have beaten Mody in Hide n' Seek he will reveal to you that under the Cavern is an underground chamber, Mody will climb through a small hole in the side while he directs you to dig up a newly revealed pile of dirt, you'll need a Spade or a Lizard character with you, digging it up will reveal an ancient hatch and climbing down it will bring you to the Forgotten Cell where you will find Mody and Lord Withermoore. Discovering the Forgotten Cell also grants the party 720 Experience.