Elemental Coalescence

You target a creature that you can see within range and choose one of the following damage types: acid, cold, fire, lightning, poison, thunder. The creature must succeed on a Charisma saving throw against your spell save DC or be affected by the spell as the cloud of the elemental energy coalesces around its body.
During your turns while you concentrate on this spell, you can force the creature to make a Dexterity saving throw as a bonus action. It takes 3d8 damage of the chosen damage type on a failed save, or half as much damage on a successful one. As a reaction when the affected creature attacks or is attacked, you can direct the elemental energy at either the attacker or the attacked creature, forcing it to make a Dexterity saving throw. It takes 3d8 damage of the chosen damage type on a failed save, or half as much damage on a successful one. 
A creature can use it's reaction to willingly fail on the initial saving throw, embracing the elemental energy. If it does so, it can use its reaction after it hits a creature to direct the elemental energy at it, forcing it to make a Dexterity saving throw. It takes 3d8 damage of the initially chosen damage type on a failed save, or half as much damage on a successful one.

The creature repeats the Charisma saving throw at the end of it's turn, ending the effect on itself on success. A creature can choose to willingly fail this saving throw. Before it makes the saving throw, it takes 1d8 damage of the chosen damage type.
At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the damage you deal with your bonus action increases by 1d8 for each slot level above 3rd.