Fiendish Soul Sorcerer

The fiendish soul embraces fully the power of the lower plains.  Your ancestor or perhaps the fiend you worked with did some unspeakable thing to your connection with magic, allowing you to access the raw fire of the lower plains, be that the Abyss, Gehenna, or Baator.  Over time you will learn to harness this power.  At first it only manifests in subtle physical changes, perhaps you grow a beard you cannot shave.  Your fingernails grow long with no explicable reason.  Perhaps more drastic changes start to manifest, horns sprouting from your forehead.  A spiny tail, or hooked barbs come out of your back.  Whatever the case, it is clear to all that you are no ordinary mortal.  As you grow in power more of that planes energy flows into you, turning you into an avatar of fiery doom that will surely terrify your foes.

Fiendish Weapons

Fiendish weapons
At first level, you gain natural weapons you may use to attack your foes. These weapons deal 1d4 damage, they are light melee weapons that count as magical. These weapons may manifest in a number of ways choosing from the following.

1d4

1. a spear made of bones drawn from your very own arms, dealing piercing damage
2. claws extend from your fingers dealing slashing damage.
3. Horns sprout from your head, able to bash foes dealing bludgeoning damage.
4. A tail extends from your lower back, with a stinger on the end, dealing piercing damage.

Or work with your DM to determine an appropriate weapon and damage type.

Brimstone Power

Starting at 6th level, when you cast a spell that deals fire damage you can add your Charisma score to one damage roll of that spell. At the same time, you can spend 1 sorcery point to gain resistance to fire damage for 1 hour.

Fiendish Transformation

Starting at 14th level, you have fully embraced your fiendish bloodline. As a bonus action on your turn you can cause yourself to become the very image of a warrior in the blood war. The damage from your fiendish weapon increases to 1d6, and you gain resistance to bludgeoning, piercing, and slashing damage from non-magical weapons that aren't silvered. You retain this transformation until you dismiss it as a bonus action on subsequent turns. For one hour afterwards, you smell of brimstone as the scent of the lower plains sticks to you.

Fire Aura

Starting at 18th level, when you undergo your fiendish transformation, hellfire flames accompany you wherever you go. At the start of each of your turns for 10 minutes after you transform, each creature within 5 feet of you takes 3d6 fire damage, and flammable objects in the aura that aren't being worn or carried ignite. A creature that touches you or hits you with a melee attack while within 5 feet of you takes 1d6 fire damage.