Costo's Glass Shield

You touch a willing creature, encasing it with an spectral glass shield. Whenever the creature takes damage the shield shatters into a smaller version of itself absorbing some of the damage, which is reduced by 1d12 + your spellcasting ability modifier to a minimum of 0. After damage is reduced in this way, the die decreases by one size (d12>d10>d8>d6>d4). After the d4 is rolled, or an hour passes, the creature loses the shield and the spell ends.