Crystal Coffin

Four sides, a top and a bottom shoot up from the ground entombing the target. The target of this spell must make a Dexterity saving throw vs a DC of 18 or be entombed in the crystal coffin. While entombed the creature is considered restrained. The coffin is immune to non magical attacks and resistant to magical Fire, Lightning, and Radiant damage. 

The crystal coffin has a Armor Rating of 18 and each side has Hit Points equal to the 1/4 of the creature entombed within. When the Hit Points of a side is expended that side shatters into crystal dust. The coffin contains 1 minute of air, after that minute if the creature is still entombed within they begin to suffocate. While a creature is entombed each point of damage the coffin takes is transferred to the creature.  If a creatures Hit Points are reduced to 0 while entombed they being to turn into crystal. After 2 rounds the transformation is complete. A minute after the creature is transformed into crystal the creature emerges from the coffin as a newly formed Crystal Golem under the complete control of the caster. A Greater Restoration spell and a Resurrection or some other kind of reviving from the dead spell will restore the creature back to their original state before the transformation.

The spell ends when the caster of the spell either moves out of range, cancels the spell, or has their concentration broken.

Only 1 Crystal Golem can exist at a time and will shatter into dust with-in 24 hours after it's creation.

The GM has the Crystal Golem statistics.