Eran's Elemental Aura

When you cast this spell, choose an element: acid, lightning, fire or cold. A shield surround yourself and up to 5 allies in a 30ft radius. Every creature affected by the shield have resistance to the chosen element.

In addition, whenever a creature within 5 feet of you or an affected allies hits you with a melee attack, the shield erupts. The attacker takes 2d8 damage from the element the shield is made of.