Crushing Prison

You create a cage of force that encloses a 40 cube, centred on a point you choose within range.  If a wall of the cage cuts through a creature's space, that creature takes 4d10 force damage and must make a Dexterity saving throw.  On a failed save, a creature is moved to the nearest unoccupied space inside the cage, or is moved to the nearest unoccupied space outside the cage on a success.

Each side of the cage shrinks by 5 feet at the end of each of your turns, unless you use a bonus action to prevent it.  If the cage wall passes through a creature's space, that creature must make a Strength saving throw, taking 4d10 force damage and being moved back 5 feet to the nearest unoccupied space on a failed save, or half as much damage and isn't moved on a success.   

The cage must have sufficient free space inside for it to shrink.  If there is a single creature inside the cage and the cage would become too small for them, that creature must make a Strength saving throw, taking 10d10 force damage on a failure, or half that damage on a success.  If there are multiple creatures inside the cage with no room to move to, then those creatures are crushed together.  Each creature inside the cage must make a Strength saving throw, taking 10d10 bludgeoning on failure, or half that damage on a success.  The spell then ends.

Any creature that starts their turn inside the cage and attempts to move through the cage wall takes 4d10 force damage and must succeed on a Strength saving throw or stop moving for that turn.  A creature inside the cage that hits a wall due to forced movement from a spell or similar effect, takes an additional 4d10 force damage, in addition to any other damage they may have taken.  A creature may move freely into the cage at time, even from forced movement.

The walls of the cage are immune to all damage.  Creatures outside of the cage cannot be affected by attacks and spells that originate from creatures inside the cage, but can still target creatures inside the cage with attacks and spells as normal.

The cage can still be affected by dispel magic, cast from either inside or outside the cage. A disintegrate cast at a cage wall removes a 10 foot cube section of the wall, which can then be passed through as normal.