Amyntall's Curious Cage

A Cage made of Light appears before you within range. The cage is a 30 foot cube. It lasts for the duration. If the cage cuts through a creature's space when it appears, the creature is pushed to one side of the wall (your choice which side).  Any creature trapped within the cage at the beginning of their turn must pass a Wisdom saving throw or be polymorphed into a tiny creature of the player's choice for 1d4 rounds.  While polymorphed, the creature does not need to make more Wisdom saving throws and is not affected by polymorph again.  

Nothing can physically pass through the wall in the same plane unless a teleport spell or similar spell is used