Comet of Cassandora

You call upon the heavens to send an destructive force to come crashing down on your foes. However, it will take time to arrive. 

When you cast the spell, you place a marker on a spot within range. At the beginning of your next turn, roll a d6. On the roll of a 6, the comet comes crashing down centered on the marker. Every creature within 30 feet of that point must pass a Dexterity saving throw against your spell save DC. On a failure, the targets suffer 7d8 force damage. On a success, they only take half of that damage.

If you roll anything other than a 6, the comet is still on its way and nothing happens. Roll another d6 at the beginning of every of your turns. For every additional turn, add one to the result of your d6 (+2 on the third turn, +3 on the fourth, etc.). Also for every additional turn, the comet deals an additional d8 of damage, up to 12d8.

On each of your subsequent turn, you can use your bonus action to move the marker 15 feet in any direction.