Deshell is a magic spell in Final Fantasy VII Rebirth. It is the second spell provided by 
Enervation Materia. It inflicts the 
Deshell effect, which reduces the 
Magic Defense of the _target, thus increasing their vulnerability to magic damage.
The spell is opposed to 
Shell, which grants the 
Shell effect that improves magic defense attributes. As such, casting it on a _target with shell will remove shell rather than apply deshell. The spell also has a physical damage equivalent, Deprotect.
Deshell is also used by Two Face.
Use
Deshell costs 8 MP to cast. It inflicts an offensive status that reduces magic defense attributes for its duration. The spell is a great means of improving magic damage against enemies and bosses, and boosting the magic damage of the party. When the player relies heavily on magic damage output, this can be an effective spell to cast.
Deshell someone competes with Faith, which boosts the magic damage attributes of a party member. Casting Deshell on an enemy can be a good way to improve the magic damage output of an entire party against that enemy, as opposed to boosting one party member's damage output potentially against multiple enemies. Alternatively, players can cast both Faith on a party member and Deshell on an enemy to maximize magic damage against that enemy.
Because deshell is opposed to shell, casting deshell will remove a shell effect but not apply shell to replace it. This has a cheaper MP cost than Dispel, unless one wishes to remove multiple statuses at once.
Deshell can be cast on all enemies enervation materia is paired with 
Magnify Materia to improve magic damage output against all enemies. In this case however, it is normally preferable to simply use Faith on a party member.