Sand Coffin

You manipulate 16 kg of sand, creating a coffin that can envelop a creature up to Huge size. The creature must make a Dexterity saving throw. On a failure, it becomes trapped inside the coffin, which is in total darkness, unable to hear anything from outside, and has a limited air supply. The coffin is invulnerable to all damage types except true damage, has 500 hit points, and regenerates 10 hit points per turn. The coffin and its interior are immune to spells, except spells that deal true damage. A creature inside the Coffin has 1 minute of air, after which it follows the rules for Suffocation.