The ways of Abyssal corruption are as infinite as the depths of the Abyss - whether from being conceived or raised in a cult, possessed as a child, or even seeking out demonic power through research and ritual as an adult - the Abyssal sorcerer draws magical energy directly from the depths of that chaotic and evil realm.

Abyssal

At first level, you can speak, read, and write Abyssal. Additionally, whenever you make a Charisma check when interacting with demons, your proficiency bonus is doubled if it applies to the check. You are permanently under the effects of a Protection from Evil and Good spell, which does not apply to fiends. You gain resistance to poison damage and are immune to the poisoned condition and are immune to negative effects from disease. You can still be infected with a disease and spread it to others, but will suffer no ill effects and show few outward signs of being diseased. 

Aura of Misfortune

At 6th level, you warp reality around you to cause destruction and misfortune to your benefit. Any creatures you choose within 10 feet suffer a penalty to their saving throws equal to your Charisma modifier. Any time a creature effected by this ability fails a saving throw within the aura, you gain advantage on your attack rolls, ability checks and saving throws until the end of that creature’s next turn.

Demonic Manifestation

At 14th level, as a bonus action, you cause bat-like leathery wings to sprout from your back. The wings grant you a flying speed of 20, or add 20 to your flying speed if you already had one. While your wings are exposed, your Aura of Misfortune ability extends to 30 feet. You can hide your wings as an action. You can't manifest your wings while wearing armor unless the armor is made to accommodate them, and clothing not made to accommodate your wings might be destroyed when you manifest them. You also gain resistance to bludgeoning, piercing and slashing damage from non-magical weapons. 

Demonic Transformation

At 18th level, you call forth the corrupting influence of the Abyss, transforming yourself into a Demon of your choice (CR less than your sorcerer level) for one minute. You gain all the physical attributes (attacks, armor class, saves, skills, resistances, immunity, etc) of the demon, but cannot cast spells and spell-like abilities in demon form, unless you also know and cast those spells as a sorcerer.