Bone Shield

Touch a friendly willing creature. A shield of whirling bones surrounds them. They gain +1 to their AC and gain resistance to all damage until they are directly hit. When they are hit, the shield shatters, dealing 4d6 piercing damage and 1d10 necrotic damage to enemies within 15 feet of the creature. 

For each spell level higher this is cast with, the target gains an additional +1 to their AC, and the shield does and additional +1d10 necrotic damage.