Elvo's Floating Minefield

You summon a 30-foot cube of small floating mines within range. When the spell is first cast creatures in the area or when a creature moves 5 feet through the area it must make a Dexterity saving throw. Roll a d8 to determine the damage type: acid, bludgeoning, cold, fire, force, lightning, piercing or thunder. On a fail the creature then takes 2d8 of that damage type or half as much on a success. That square of mines is then destroyed. The creature can decide to move at half speed to not take any damage while moving. 

Each square of mines has 1d4 hit points and an armor class of 20. The mines can be targeted by weapon attacks and spells, including area of effect. The mines are immune to psychic and necrotic damage. Once a square of mines reaches 0 hit points the mines detonate and any creatures in the square must make a Dexterity saving throw.

Creatures smaller than medium have advantage on the saving throw, and creatures larger than medium have disadvantage. Whenever a creature larger than medium moves through the area and it rolls lower than spellcasting modifier on its Dexterity saving throw another square of mines that it is in detonates.

At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d8 for each slot above 3rd. Alternatively, you can increase the size of the cube by 5 feet for every slot above 3rd.