Armina's Crystal Vitrine

Una Armina - renown Madwoman, Artificer and curator of the Museum of Oddities - was famous for her fine collection of strange, wonderful and sometimes horrific artifacts, yet as the glassmaking methods of her time were highly limited her displays often cost more than the artifacts themselves! Thus after years of research, Armina's Crystal Vitrine was born. While only created to display and protect her fine collection of oddities from grubby hands, the versatility of this spell lends to some unique combat utility as well..

Crystalize the air around the target, creating an elegant clear glass box around a visible object/creature of medium size or smaller within range. The glass is transparent, 1/4 inch thick and the structure is roughly sized to the dimensions of the target. The structure does not have a bottom but is lightly affixed to the surface the target is resting upon, and can be loosed by the occupant as a bonus action and then moved at half speed. If concentration is maintained for the full minute the structure becomes permanent, if the structure was not moved during the full minute it becomes bonded to the surface and can not be lifted without breaking it. If concentration is broken or released before the minute is up, the structure disappears without shattering. 

When any part of the glass is broken the entire structure shatters creating a loud, satisfying sound and dealing 1d4 slashing damage to the creature within due to falling glass. Additionally, if any creature uses their body to break the glass (punch, kick, thrown into it, etc. as opposed to attacking with a weapon or projectile), that creature takes 1d4 slashing damage.

The structure must be destroyed before any effect which requires line-of-effect can be used to target the contents (full cover).

The structure is delicate with 1HP, and can be shattered if hit by any attack that could damage an object (unaffected by poison damage, etc. per DM judgment). A creature may use its opportunity attack (if applicable) to targeting the structure.

"I'm always looking for new exhibits, so remember me when you come across anything Odd." ~ Una Armina

Example: if a creature inside punches the glass, they will take a total of 2d4. If a creature outside punches the glass, that creature will take 1d4 damage and the creature inside will take 1d4. If a creature uses its body for their opportunity attack (not a weapon) to break through the glass they take 2d4 damage.

Additional DM Notes:

Utility: Unavoidable. Can be cast on an item to make a sweet display (nifty anti-theft utility). Forces a creature to spend its opportunity attack to break the glass, or action/bonus action if unable to use reaction. Acts as full cover if cast on allies (and enemies) allowing them to stay safe from most single attacks at the risk of 1d4 damage and limiting attack options/movement. In the right scenario can allow one to avoid a reaction (and take or deal some damage in the process). This spell is best used to assist in disengaging from combat against a single strong enemy, has many potential niche uses outside of combat. Damage does not scale (which is counterintuitively a way of scaling the spell since it becomes more useful for defense as the damage becomes more trivial).

Balancing: Takes an action and a spell slot for the caster in combat. Requires concentration. *Most creatures will be able to use a reaction/opportunity attack to destroy the glass the moment it is cast*. If not able to use a reaction, almost any action or bonus action will break the glass which means breaking almost always requires less action resource than casting. If used for allies, they are effectively disabled from combat as long as they remain inside since most ranged abilities can not be used from inside the box (even though visibility is not limited, full cover means any attack which requires the target to be accessible/requires line of effect is not valid while the structure is intact). Only ever limits action for at most one round (assuming any attempt is made to break the glass). The damage is a slight risk to the player at early levels and there are many better ways of disengaging from combat or avoiding a blow. damage does not scale, and utility in an emergency is outclassed by most spells actually intended for that purpose.

Mechanical notes: The box could be interacted with like any physical box, ie: could be dug under on an appropriate surface, could be fully lifted and moved if small enough (dependent on the size of the object/creature it is cast upon) or with outside assistance.