Elemental Rune

You draw a magic rune upon a surface with in range. Upon drawing the rune you choose acid, cold, fire, lightning, poison, or thunder for the type. The rune is nearly invisible and requires a successful Perception (Wisdom) check against your spell save DC to be found. If any creature that is hostile to you enters the area that the rune is in, the rune then triggers, that creature and any other creature with in 5 feet of the rune, must make a Dexterity saving throw. Each creature takes 3d6 damage of the type you chose when drawing the rune on a failed save, or half as much on a successful one. 

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher the damage increases by 1d6 for each slot level above 1st.