Catlana's Crustacean Carapace

This spell appears to be a modified version of Mage Armor and can't be prepared or learned by itself. Instead, knowing or having the spell Mage Armor prepared allows someone who has studied this spell the option to upcast Mage Armor as Catlana's Crustacean Carapace.

You touch a willing creature who isn't wearing armor, and a hard crustacean carapace magically forms around their torso. Their arms, if they have any, develop a similar shell and claws form over their hands.

The target's base AC becomes 15 + its Dexterity modifier, and they have disadvantage on using weapons or tools held in their claws. Their claws are natural weapons which can be used to make unarmed strikes. The damage from their unarmed strikes is slashing instead of bludgeoning, deals 1d4 + their Strength modifier and is considered magical. The target of their unarmed strike automatically becomes grappled as long as they have a free hand to do so.

The spell ends if the target dons armor or if you dismiss the spell as an action. If the spell ends because the duration runs out, the shell molts away from their body and they gain the effects of Mage Armor as though you had cast it and for the 8 hour duration of Mage Armor.