Board Thread:Questions and Answers/@comment-82.211.213.143-20131216140848/@comment-50.157.97.249-20140506012306

That was a Monolith. They are placed outside of "closed" dungeons - dungeons with a definite "inside" and "outside" to keep players inside. This is to prevent easy circumvention of traps. If they see you, they will send you to Limbo. They also exist in Limbo. In Limbo, they send you to a random part of Limbo, thus impeding progress to the Ancient Fabric (the blocks that send you back to the Overworld). Sometimes, they spawn inside the "inside" of a dungeon as a glitch, in which case I would leave the dungeon immediately, and mark the door leading directly to the Monolith-infested dungeon as "dangerous" with a sign. Also, bring an Ender Chest so if you do end up in Limbo, you can put all of your items into the Ender Chest, and upon reaching the Overworld, use the command /kill to return to your spawn/bed (and presumably your settlement)  and use another Ender Chest to retrieve your items.