The Floor is Lava

Lava fills a 20 foot square centered on a point that you can see to a height of 1 foot. The lava lasts for the duration and sheds bright light in a 20-foot radius and dim light for an additional 20 feet. It also ignites all flammable objects within 5 feet of it that aren't being worn or carried.

All creatures within the square must succeed on a Dexterity saving throw or take 4d6 fire damage. A creature who fails the initial saving throw must repeat the saving throw on each of its subsequent turns until it succeeds or the spell ends, taking an additional 2d6 fire damage on each failed save. Creatures have advantage on the saving throw if there are grounded objects nearby. In addition, you can select up to two creatures (including yourself) to automatically succeed on the initial saving throw.

At Higher Levels. When you cast this spell using a spell slot of 5th level or higher, you can either increase the dimensions of the square by 5 feet or increase the damage from each failed save by 1d6 for each slot level above 4th.