The Penance of Protection

A cage of broken iron and motes of power surround the creature, protecting them against physical damage. The target gains +10 to their AC, until hit by an attack or the duration of the spell ends. When hit by an attack they take damage as though they were vulnerable to that damage type as the armoured cage crushes in as a response. 

At Higher Levels. Casting this spell at 4th level or higher levels allows the caster to choose one type of damage to grant resistance to if the spell fails. This resistance can be magical or non-magical bludgeoning, slashing, or piercing damage and must be declared upon casting the spell. This resistance overrides any negative effects of the spell.