Vylan's Vitriolic Pit

An extradimensional, cube-shaped, acid-filled pit appears on an area of ground that you choose within range.
The pit can be up to 10 feet on a side and lasts for the duration. When a creature or object enters the pit for the first time on a turn or starts its turn there, it must make a Constitution saving throw. The target takes 10d4+20 acid damage on a failed save, or half as much on a successful one. A creature must spend 2 feet of movement for every 1 foot it moves in the pit. A creature in the pit and adjacent to a side can use its action to make a Strength (Athletics) check against your spell save DC. If the creature’s size is greater or equal to the size of the pit, it has advantage on this check. On a success, it frees itself.
When the spell ends, the acid-filled pit disappears and creatures inside are harmlessly ejected onto the surface. Additionally, the ground in that area becomes difficult terrain.
At Higher Levels. When you cast this spell using a spell slot of 7th level or higher, you may increase the size of the pit by 5 feet on each side for each slot level above 6th.