Invocation: Elemental Overflow (UA)

Prerequisite: Level 5+ Warlock

Choose a damage type: Acid, Cold, Fire, Lightning, or Thunder. Whenever you cast a spell that deals the chosen damage type, you can cause elemental energy to wreathe you until the end of your next turn. For the duration, whenever a creature within 5 feet of you hits you with a melee attack roll, that creature takes 1d4 damage of the chosen damage type.

Repeatable. You can gain this invocation more than once. Each time you do so, choose a different damage type.