Booming Rune

A glowing rune encased in stone flies up from the ground in front of the target. The target takes 1d12 Thunder damage, and at the start of each of your turns you can use your action to command the rune to boom again dealing another 1d12 Thunder damage. the spell ends if you use your action to do anything else, the target uses their action to destroy the rune. (the rune has no ac and 1hp) or if the target leaves the range of the spell.

At higher levels: the initial damage increases by 1d12 for every level above 1st