Bigby's Acid Rain

You reach out with your hands and large green clouds form over a 25 ft. by 25 ft. area as drops of acid starts raining down.
Each creature within the area must make a Constitution savingthrow.
On a failed save, a creature takes acid damage equal to 1d8 + your spellcasting ability modifier, or half as much damage on a successful save.
A creature takes the damage if its in the area when the clouds first appear and when it finishes or starts it's turn there.
Additionally for every time a creature moves 5 ft. within the area, the creature takes 1d6 additional acid damage.
The spell is active as long as the caster maintains concentration on the spell up to 1 minute.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d8 for each slot level above 2nd.