The magic of the abyss courses through you.  Abyssal hellfire, Infernal magic, and more are available to you.  You may have gotten this magic from being part of a cult, the specific attention of a powerful fiend, or simply from being descended from one.

 

(Note:  If you have a tail and one of the traits gives you a tail, your tail simply changes to the tail from the trait.)

Fiendish Origin

Choose Abyssal or Infernal.  You may now speak, read and write in this language.  Furthermore, when you make a charisma check with a fiend that speaks this language, you do so with advantage.

Abyssal

Your skin turns a dark shade of red.  Your hands grow long, curved claws, which are natural weapons, which you can make unarmed strikes with, dealing 1d4 + your Strength modifier on a hit.

Infernal

Your eyes look unnatural and you have slitted pupils.  You grow a tail like an imp's which is a natural weapon that you can make unarmed strikes with, dealing 1d4 + your Strength modifier piercing damage on a hit, instead of the bludgeoning damage normal for an unarmed strike.

Unholy Flames

Starting at 1st level, your power over Brimstone flames has begun.  Once per turn, you can alter a Sorcerer spell you cast to burn your enemies more furiously, based on your Fiendish Origin

Brimstone Fireblast

Once per turn, you can increase the fire damage of any Sorcerer spell you cast by your proficiency bonus.

Nine Hellsfire

Once per turn, you may ignore a creature's resistance to fire damage with any Sorcerer spell you cast, unless the creature is a fiend.

Brimstone Affinity

Starting at 6th level, you gain powers close to those of the fiends you resemble.  You gain resistance to fire damage.  You have advantage on saving throws against spells and being charmed if you already have resistance to fire damage.  In addition, when you cast a spell that deals fire damage, you spend 2 sorcery point to cast it at one level higher.  You cannot cast a spell at a level higher than 9th with this feature.

Fiend Traits

At 14th level, you gain the ability to sprout a pair of fiend wings from your back, gaining a flying speed equal to your current walking speed.  While these traits are manifested, your natural weapons from your fiendish origin deal an extra 1d10 poison or fire damage on a hit (Your choice when you hit).  You can create these traits as an action on your turn. They last until you dismiss them as an action on your turn.

You can’t manifest wings while wearing armor unless the armor is made to accommodate them, and clothing not made to accommodate your traits might be destroyed when you manifest them.

Brimstone Power

Beginning at 18th level, you can channel the dread power of the lower planes.  Spells you cast ignore resistance and immunity to fire damage.  In addition, once per turn you may increase the damage of any Sorcerer spell you cast that deals fire damage by your level.