Tasha's Expanding Cauldron

A puddle of miasmic acid forms on the ground centered on a point you can see within range. There must be a roughly flat and solid floor for the entirety of the puddle to form, otherwise the spell fails. The puddle is 20 feet in diameter and and a few feet deep, and the area within is difficult terrain. A creature that starts its turn in the puddle or enters it for the first time on a turn must make a Constitution saving throw. They take 10d4 acid damage on a failed save, or half as much on a successful one. 

While concentrating on this spell, you may use an action to increase the size of the puddle by 10 feet in diameter. The puddle spreads around corners and around obstacles this way, and loose objects fall into the puddle as it expands. If there is no more surface for the puddle to expand across, such as against a wall or over a cliff, the puddle stops growing and cannot be expanded further.

At Higher Levels. When you cast this spell using spell slot of 6th level or higher, the damage increases by 2d4 for each slot level above 5th.