Basic Hellfire

You create a cylinder of darkened flames from hell. Pick a point you can see within range. You create a 20 foot radius cylinder, up to 30 feet high of writhing black and red flames. Every creature within the cylinder must make a CON save. On a fail, they take 3d8 fire damage and 3d6 necrotic damage, and they catch on fire in a black, hellish flame. At the start of their turn, they take 1d6 necrotic damage. This flame affects them even if they are on fire from another affect, such as a Fire Elemental's Touch attack. It takes an action to put out the flame. On a success, the creature takes half damage and is not caught on fire. The fire spreads around corners. It ignites flammable objects in the area that aren’t being worn or carried.

Any creature doused in holy water or holy oil can not be set on fire by the hellish flame and has advantage on the saving throw. 

At Higher Levels: For every spell level above 3rd level, increase the damage by 1d8 fire and 1d6 necrotic damage.