Cube Canon

You conjure a fist-sized cube that you hold in one hand. The cube is transparent except for a solid blue outline. After one round, a small red ball forms within the cube and bounces around. The ball grows bigger at the start of each of your turns until it completely fills the cube after a total of six rounds. At any time on your turn, while the ball is in the cube, you may use an action to release the ball from its container. The ball targets the nearest creature other than you. That creature must make a Dexterity saving throw or take an amount of force damage depending on the ball's size: 1 round, 5d10; 2 rounds, 6d10; 3 rounds, 7d10; 4 rounds, 8d10; 5 rounds, 8d12; 6 rounds, 8d12. If the ball was released after six rounds then it returns to the cube as the smallest ball and this spell resets as if you had just cast it (this resets duration). If a creature succeeds the saving throw then the ball becomes half its size and returns to the cube.