You create a 10ft Cube of Darkness with cube of lightning within it. Creatures within the area must make a Dex Saving throw or be caught within the cube.

On a successful save, they dodge the effect but still take half of 2d12. On a failed save they are caught within the cube and blinded. Each of your turns, all creatures in the cage take 2d12 lightning damage. 

Creatures may attempt to leave the cube on their turn with a strength saving throw at disadvantage. If a creature escapes the cage they take a flat 24 damage.

At Higher Levels: Each level above 4th that this spell is cast, does an additional d12 each turn. The flat damage on escaping creature increases by a flat 12 as well.