Thistles colorful aura

You create an aura of piercing, fire, cold, lightning, acid or thunder in a 10 foot radius around the caster, whenever a hostile creature enters the aura

(approaching a hostile creature also counts as entering, however they cannot take the damage more then once per round)

they have to make a dexterity saving throw or take 2d4 damage of the chosen element or half as much on a successful save.

 At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the initial damage increases by 1d4 for each slot level above 1st.