Elemental Mine

When you cast the spell, choose acid, cold, fire, or lightning. You create a magical energy trap that takes the form of a tiny glowing rune the size of a fingerprint. The trap erupts either when a creature other than the caster enters within 5 ft. of the rune or when the caster spends a bonus action to manually erupted. When it erupts, it deals 4d6 damage of the chosen elemental type to all creatures within a 20 foot radius of the rune. Once the trap erupts the spell ends.