Crystal Citadel

The  is a universe that has held the most dangerous criminals of the Multiverse for eons. It was not mentioned or talked about in the episode, "The Lich," but it is a possibly unproduced place that was talked about in the storyboard for it. It is very easy to enter, but nearly impossible to escape due to the strange spacial physics of the dimension. It is also one of the most accessed universes. According to the storyboard, Finn's real dad (Ooo) was unlawfully imprisoned in the Crystal Citadel where he battles the criminals within to this day.