The Inverse Tower

From the introduction:

The Inverse Tower is simultaneously a dungeon, magic item and portal. It will appear in the campaign world when and where the GM sees fit. The reward for besting the tower is entry to the inverse world, a plane that manifests as the exact opposite of whatever the GM has mapped and written to exist in the campaign above. All actions taken here magically have the opposite effect on the world above (Fires cause floods, deaths cause births, lost things are found). Do not let the adventurers know this until they have returned to their once “normal” world...