Fiendish Soul

Occasionally, a soul is born with ties to the infernal. Those bearing such a soul often find themselves with innate magic flowing through them that draws back to their hellish origins. Those with fiendish souls are often the offspring of warlocks who have made pacts with devils or demons, or those who have fallen in love with and raised children with a fiend. Rarer still are children born with souls that have been touched by some unknown, infernal force, granting them kinship to the nine hells or the abyss.

Those with fiendish souls are often discriminated against and hunted by the more fervent of religions, being seen as "impure" and "accursed". But in some cultures, children with innate devilish magic are considered to be heralds and prophets, and are reveared and elevated for their power. However, merely being in possession of a fiendish soul does not detirmine the alignment of the bearer: Those with infernal magic flowing through their veins can be just as good, or just as evil, as any others.

Infernal Magic

Your link to the infernal allows you to learn an expanded range of spells normally reserved for and fiendishly devoted clerics and warlocks. When your Spellcasting feature lets you learn or replace a sorcerer cantrip or a sorcerer spell of 1st level or higher, you can choose the new spell from the expanded spell list or the sorcerer spell list. You must otherwise obey all the restrictions for selecting the spell, and it becomes a sorcerer spell for you.

Luck of the Devil

Starting at 1st level, you have uncanny luck thanks to your infernal heritage. If you roll a critical failure on a saving throw or attack roll, you can roll 3d6 and replace the outcome with the new number. A roll of 18 (three sixes) counts as a critical success, however a roll of 3 (three ones) counts as a critical failure. Otherwise, normal skill and attack bonuses are added to the total number rolled. This ability can be used once per long rest.

Hellish Surge

Starting at 6th level, the infernal power coursing through you can empower fire and necrotic based magic. Whenever you roll dice to determine the fire or necrotic damage a spell deals to an enemy, you can force one of those dice to be a maximum roll, provided you aren’t incapacitated. You can use this feature only once per short rest, and it can be used in conjunction with metamagic.

Infernal Form

Starting at 14th level, once per short rest you can use a bonus action to manifest an infernal change to your body. You have the choice between devilish wings, large horns, sharpened talons, a whip-like barbed tail or infernal, blazing eyes. The effects lasts one hour, or until you’re incapacitated, you die, or you dismiss them as a bonus action.

If you naturally already have one of these bodyparts (I.E horns or a tail) then you can either choose to grow an additional bodypart alongside your existing one, or you can have the existing bodypart change to reflect the ability for its duration.

Eyes

Your eyes glow with blazing infernal magic, granting darkvision up to 100 feet, immunity to being blinded, and +1 to all perception rolls.

Horns

You grow large, pointed horns from your head that allow you to headbutt opponents within 5 feet as a bonus action, dealing 1d8 plus your strength modifier piercing damage.

Tail

A long, barbed tail grows from the base of your spine that can be used to strike opponents within 10 feet as a bonus action, deals 1d6 plus your dexterity modifier slashing damage.

Talons

Long, razor-sharp talons extend from your fingertips that allow you to slash at opponents within 5 feet as a bonus action, deals 2d6 plus your dexterity modifier slashing damage.

Wings

You sprout leathery, bat-like wings from your back that grant the ability to fly with flying speed of 30 feet.

Fiendish Might

At 18th level, your fiendish power reaches a peak, flooding your body with powerful infernal energy. You automatically learn the spells burning hands and hellish rebuke and become able to cast them at will without expending a spell slot. The spells count as 1st level spells when cast. Your infernal nature also grants you fortitude against these same hellish energies, allowing you to become resistant to both fire and necrotic damage, as well as granting you advantage on any saving throws against these damage types.