Fiendish Bloodline

Sometimes the spark of magic that fuels a sorcerer comes from a divine source that glimmers within the soul. Having such a blessed soul is a sign that your innate magic might come from a distant but powerful familial connection to a divine being.
This isn't that at all.

Your magic comes from the darkest planes in the multiverse. Maybe your bloodline can be traced to a triste between mortal and a fiendish creature. Maybe your parents sold their firstborn (that's you) to a devil for power, only to realise they never specified who would be granted that dark gift.
Tieflings are feared out of juvenile superstition. You are quite possibly viewed the same way, but in your case, the suspicion is likely well-earned. Whether you like it or not, the dark presence of the Abyss, Gehenna, or the Nine Hells is one that will never leave your side.

Neat.

Profane Magic

Your involuntary ties to the Lower Planes allows you to learn spells normally associated with the warlock class. When your Spellcasting feature lets you learn a sorcerer cantrip or a sorcerer spell of 1st level or higher, you can choose the new spell from the warlock 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.
In addition, choose an affinity for the source of your fiendish power: The Abyss (demons), Gehenna (yugoloths), or the Nine Hells (devils). You learn an additional spell based on that affinity, as shown below. It is a sorcerer spell for you, but it doesn't count against your number of sorcerer spells known. If you later replace this spell, you must replace it with a spell from the warlock spell list.

Affinity	Spells
The Abyss	Cause Fear
Gehenna	Hex
The Nine Hells	Charm Person

 

Fiendish Visage

Starting at 1st level, your lineage has left its mark on your appearance, a visual abnormality that brings a sense of unease to those who see it. You decide what the mark is, or you can determine what it is randomly by rolling on the Fiendish Visage table.

d6	Fiendish Mark
1	Slitted eyes
2	Curled horns
3	Split, snake-like tongue
4	Rough scales surrounding your features
5	Sharp fangs that extend past your lip
6	All of the above

While your Fiendish Visage is visible, you have advantage on Charisma (intimidation) checks made against any creature that is not a Celestial or Fiend, and disadvantage on Charisma (persuasion) checks against any creature that is not a Fiend. Creatures charmed by you are unaffected by this feature. Additionally, you can read and write Abyssal or Infernal (you choose when you gain this feature). You can also speak the chosen language, but only while your Fiendish Visage is visible.

As a bonus action, you can hide your fiendish Visage, making your face appear not to possess its unusual qualities. You can mask your Visage in this way until you lose concentration (as if concentrating on a spell). 

Those who witness your fiendish form might react poorly to your true face, depending on their original disposition towards you and their knowledge of fiends.

Curse of the Forsaken

Also at 1st level, the dark powers that encompass your being can be used to balefully smite those around you. As an action, you can target a number of creatures up to your Charisma modifier within 60 feet of you. Those creatures are cursed for 1 minute. Whenever a cursed creature fails an attack roll or saving throw, that creature takes fire or necrotic damage (your choice) equal to the amount by which it failed the attack roll or saving throw, up to a maximum of twice your sorcerer level.

You can use this feature a number of times equal to your proficiency bonus, and regain all expended uses when you finish a long rest.

Malevolent Form

At 6th level, your lineage is more apparent in your form, so much so that your tainted blood can be used for rituals that would otherwise require greater sacrifice.

While your Fiendish Visage is visible, you can see normally in darkness, both magical and nonmagical, to a distance of 60 feet. You also gain benefits based on your Profane Magic feature, having taken on greater aspects of your supernatural heritage:

• Abyssal Savagery (The Abyss): Your skin hardens into rough scales. Your AC equals 13 + your Dexterity modifier. You can spend 2 sorcery points when you use your Curse of the Forsaken feature. If you do, any creature affected by the curse takes double the damage it usually would by missing you with an attack roll.
• Chthonic Spite (Gehenna): Your skills are enhanced by the downfall of your enemies. You have advantage on Charisma (Deception) checks on any creature except fiends. You can spend 2 sorcery points when you use your Curse of the Forsaken feature. If you do, you gain advantage on the next ability check or saving throw you make before the end of your next turn whenever a creature affected the curse misses an attack roll or fails a saving throw.
• Infernal Charm (The Nine Hells): Your voice becomes supernaturally soothing and suggestive. Creatures except fiends and those with truesight must make a Wisdom (Perception) check against your sorcerer Spell Save DC in order to perceive your Fiendish Visage. You can spend 2 sorcery points when you use your Curse of the Forsaken feature. If you do, creatures affected the curse have disadvantage on saving throws made to resist being charmed by you. Damage done to said creatures by the curse does not affect the charmed condition on them.

Additionally, when you cast a Conjuration spell of 1st level or higher that requires the blood of a humanoid killed within the last 24 hours as a material component, you can instead choose to take necrotic damage equal to 1d4 per level of the spell slot expended, which cannot be reduced in any way, using your own blood for the casting.

Fury's Flight

At 14th level, you gain the ability to sprout a pair of leathery, fiendish wings from your back, gaining a flying speed equal to your current speed. You can create these wings as a bonus action on your turn while your Fiendish Visage is visible. They last until you dismiss them as a bonus action on your turn or you conceal your Fiendish Visage.

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

Crown of Thorns

At 18th level, the authority which accompanies your vile power can be called on to summon fiends from the lower planes, using your adversaries as sacrifices for your dark conjuration.

When a creature affected by your Curse of the Forsaken feature is killed while you can see it, you can spend any number of sorcery points to summon a fiend of a challenge rating equal to or lower than the challenge rating of the slain creature, or its level if it does not have a challenge rating. Any fiend summoned by this feature exists for a number of minutes equal to the number of sorcery points spent when it is summoned, or until it is reduced to 0 hit points.

Fiends summoned in this way are an ally to you and your companions. In combat, they share your initiative count, but take their turn immediately after yours, acting as a group.