Fiendish Bloodline

Whether you're proud of it or loath it your blood carries the power of the lower planes. The blood of fiends runs through your vanes. When you were younger you may have burned down a structure or immolated a companion when you were angry. Perhaps you disturbed those around you by speaking in abyssal or infernal. Loath it or love it, at one point you accepted your power. From there on the world was a different place.

Fiendish Legacy

At first level you gain the ability to see in darkness and magical darkness out to 120 feet. You also know the spell, Burning Hands. This does not count against your number of known spells.You speak the languages of your blood allies and blood enemies.

At level 1 your heritage increases your resilience. You get an additional 1HP per level.

A fiend from the 9 Hells or the Infinite Abyss comes to you as a familiar to serve as your guide and connection to your chosen side. This does not require you to cast a spell. You can summon or dismiss your familiar as an action. If your familiar disappears because its hitpoints were reduced to 0 it is automatically summoned again after a long rest. The familiar will take the shape you choose, but must be a fiend. All other rules are the same as listed in the find familiar spell.

Summon Fiend

At level 6 you establish a contract with either a devil or a demon of CR 6 or lower that you can now summon for 4 sorcery points. You give this fiend a single command, such as attack that creature, and then it disappears when the task is complete. If the task is too complex it may require a negotiated payment from the caster. The fiend can be communicated with telepathically up to 100 ft from the caster. If the fiend is reduced to zero hit points before carrying out this task it cannot be summoned again until the caster finishes a long rest. Unless dismissed the fiend can choose to remain of its own volition on the material plane. A fiend of neutral alignment (on the lawful-chaotic axis) can be summoned by either type of fiendish sorcerer.

The contract is binding and cannot be broken unless the fiend is truly destroyed, the sorcerer dies or somehow loses this class feature, or the greater effect of the wish spell is used. 

Fiendish Wings

At level 14 your fiendish blood grows in power enough to create a pair of bat-like wings from your back as a bonus action. They give you a flying speed of 30 feet and remain until you dismiss them as a bonus action or fall unconscious. If you are wearing armor not specially made for the wings you cannot summon them. If you are wearing clothes that are not made to accommodate the wings the clothes may be destroyed when you summon them.

Fiendish Might

Your connection to the profane has grown so strong you feel it coursing through your blood. You can expend 10 sorcery points to become a demon or a devil as a bonus action. The effect is that of True Polymorph, but with the following restrictions: You can only become a demon or devil of a CR equal to or lower than your level and is not made permanent by concentrating for the duration. Additionally, it does not require concentration to maintain. The effect lasts for 1 minute.