Elemental Coffin

Guarding yourself from blades and steel, as a reaction, when a creature targets you with a melee attack, you can summon a casing of one of your alchemical elements to protect you. The shield has 1d10+20 hp. When the drops to zero it dissipates. On your next turn, if the shield is still up, you can choose to detonate the shield. Each creature within 5 feet of you must make a dexterity saving throw against your spell DC. Any creature that fails the save takes damage for the remaining health pool of said shield, of the respective damage type. . 

At Higher Levels. When you cast this spell using a spell slot of 5thlevel or higher, you add 5 additional hit points for each slot level above 1st.