As soon as the tower has been unlocked and you have reached at least level 140, you get the key to the first Shadow dungeon.
You can get to Shadow World via the the dungeon icon within the fight menu or the respective building in the city.
The enemies in the dungeons of Shadow World can be compared to the villains of Light World with an increased difficulty level because the darkness gives them incredible strength. Luckily, these dungeons are explored with your companions!
On the shadow world map you will find all available dungeons. Once a dungeon was unlocked you will find information about the next opponent, incl. level, class and assigned element.
You get access to each Shadow dungeon with a key which can be found in the gem mine, but only after at least five dungeon floors have been defeated in the previous Shadow dungeon.
Similar to the tower, you gain an epic item from each enemy in Shadow world, either for your character class or for one of the companion classes. Items from Shadow dungeons can't be thrown in the toilet and they can't be sold. You can dismantle them though.