Tiamat's Kiss

A Cloud made up of sheets of Thick black smoke appears in a location you choose within range. The area of the Cloud consists of up to ten 20-foot cubes, which you can arrange as you wish. In the area must make a Dexterity saving throw. It takes 7d10 fire damage on a failed save, or half as much damage on a successful one, each creature which starts is action in the smoke storm takes half of the rolled damage

By making this a full round  to cast the spell you can double the size and add an extra 2d10 to the damage but you are completely preoccupied casting it

The fire damages objects in the area and ignites flammable objects that aren't being worn or carried. If you choose, plant life in the area is unaffected by this spell.