Crystallizing Mage Armor

You touch a willing creature who isn't wearing armor, and protective scales form on it until the spell ends. The target's base AC becomes 13. The spell ends if the target dons armor or if you dismiss the spell as an action.

When you cast this spell again, you can choose to reset the spell's duration and increase the AC by 1. When you reinforce the spell in this way, the spell slot increases as the target's AC does, calculated by (AC - 11). The target's AC cannot increase above 13 + the target's proficiency in this manner.