The Mirror Rift is a magic portal filled with dungeons that contain 6 types of elements (Fire, Water, Earth, Basic, Light, and Dark).
Instead of normal enemies, like Invaders, the enemies are a mirror image of the heroes from the book and thus, they have all the abilities the hero has.
The dungeons have different types of heroes depending on which rift you are. Fire heroes in fire dungeons, for example. That's why it is recommended to use appropriate heroes with buffs by element; like Water heroes in the Fire dungeon.
Currently, there are 14 stages from level 60 to level 86 skipping levels with odd numbers. The main rewards in level 60 are 11 Mirror Shards and in level 86 it goes up to 39.
The special rewards also change depending on which rift you are playing. All of the dungeons have a certain chance of giving you a specific Accessory. The Fire dungeon gives you the Ruby Mirror Ring, the water dungeon gives you Mirror Earring of Worship, earth gives you Mirror Necklace of Riches, basic gives you Mirror Earring of Patience, light gives you Mirror Earring of Faith and finally dark dungeon gives you Mirror Earring of Nobility.