Located deep in the ancient caverns of the earth are hidden structures, housing the untold power of the affixes. It would be a welcome surprise if that power was free for the taking, but alas, it is instead defended by a Caged Invader.
Found underground are a few different variants of these structures. Each dungeon contains a Caged Invader and a few loot chests. The cage will trigger upon getting close to it, unleashing the Invader.