Bone Shield

The caster shields themselves in an armor of magically conjured bones. He gains +3 AC for the duration. Every time he gets hit by an attack, the bones splinter and the AC is reduced by 1. Additionally, as long as the attacker is within 60 feet he has to succeed on a dexterity saving throw or suffer 4d6 slashing damage. This spell ends prematurely when the bonus to AC is reduced to 0.

Cast at higher levels. When cast at 4th, 6th or 8th level, the bonus to AC is increased by 1 for every two levels this spell is cast above 2nd level