Deshell (弱心（デシェル, Jakushin (Desheru)?, lit. Weaken Spirit (Deshell)) is a recurring status effect that debuted in the Lightning Saga that reduces the target's Magic Defense. It is opposed to the Shell status, the two cancelling each other out.
Appearances
Final Fantasy VII Rebirth
Deshell reduces target's 
Magic Defense for 30 seconds by around 10%. It can be applied by the 
Deshell spell from the 
Enervation Materia. It is opposed to 
Shell, one nullifying the other.
Final Fantasy XIII
Deshell decreases resistance to magical damage by 89%. It can be inflicted on opponents with the Deshell and Deshellga spells. It is opposed to Shell and Shellra.
Final Fantasy XIII-2
Deshell decreases resistance to magical damage by 30%. It can be inflicted on opponents with the Deshell and Deshellga spells.
Lightning Returns: Final Fantasy XIII
Deshell decreases resistance to magical damage by 33% and may make the victim become susceptible to being knocked back by powerful attacks. Lightning can inflict opponents with Deshell using the Deshell and Deshellga spells. She can increase the status's duration with auto-abilities and by synthesizing the spells and leveling them up in Sorcery Shops.
When Fang is in the party, she will use Deshell Hunter against targets afflicted with Deshell dealing increased damage to them.
If afflicted, Lightning can heal the status with Remedy or Mega Remedy, but it will eventually expire on its own.