The Mist

The mist, otherwise called the veil, the barrier or the wall was made by the highest ranked magic users after the humans tore down everything from the past. The mist makes it so humans can't see the hidden world (also called the supernatural world or the not humans), so they rationalize advents that happen around them. It also made them forget the supernatural creatures exist, which is why we have myths, legends and other stories because its the human brains way of coping with the knowledge locked away. The only 'supernatural' creatures that are still remembered and known to exist are the deities because they made it impossible for anyone to forget them.
For example, a human thinks they miss place their keys, however in truth an imp moved them right in fount of the human. The human can't process the imp being there so they think it was them forgetting instead because its easier to think that.