Fiendspawn

Your magic is tied to the Fiendish Planes. Perhaps your ancestors were pacted with a powerful archdevil from the Hells or a demon of the Abyss, or perhaps you bear their corruption from a curse. Regardless of how you gained this magic, your blood has been tainted by the influence of the Lower Planes.

Source: Me, Myself, and I

Unholy Magic

Starting at 1st level, you learn additional spells when you reach certain levels in this class, as shown on the Unholy Spells table. Each of these spells counts as a sorcerer spell for you, but it doesn't count against the number of sorcerer spells you know. Whenever you gain a sorcerer level, you can replace one spell you gained from this feature with another spell of the same level. The new spell must be a divination or an enchantment spell from the sorcerer or warlock spell list.

Spell Level	Spells
1st	Ray of Sickness, Hellish Rebuke
2nd	Crown of Madness, Borrowed Knowledge
3rd	Bestow Curse, Enemies Abound
4th	Shadow of Moil, Sickening Radiance
5th	Cloudkill, Infernal Calling

Level 1: Mark of the Beast

Beginning at 1st level, you choose one type of fiend as the source of your power. The type that you choose is used by features you gain later.

Devil. You can speak, read, and write in Infernal. Additionally, whenever you make a Charisma check when interacting with devils, your proficiency bonus is doubled if it applies to the check.
Demon. You can speak, read, and write in Abyssal. Additionally, whenever you make a Charisma check when interacting with demons, your proficiency bonus is doubled if it applies to the check.
Yugoloth. You gain telepathic speech. To understand and be understood by other creatures, you must share at least one language. Additionally, whenever you make a Charisma check when interacting with yugoloths, your proficiency bonus is doubled if it applies to the check.

Level 1: Fiendish Vision

At 1st level, you can see normally in darkness, both magical and nonmagical, to a range of 60 feet. Additionally, when you reach 6th level in this class, your attunement to the darkness of the Lower Planes grants you blindsight with a range of 10 feet.

Level 6: Embraced Damnation

At 6th level, you gain resistance to one damage type according to your choice of fiend in your Mark of the Beast feature as well as an appropriate cantrip, which does not count against the number of cantrips you know. Additionally, when you cast a spell of that damage type, any resistance to it is ignored.

Devil. You gain resistance to fire damage and the Produce Flame cantrip.
Demon. You gain resistance to poison damage and the Poison Spray cantrip.
Yugoloth. You gain resistance to acid damage and the Acid Splash cantrip.

Level 14: Master of the Profane

At 14th level, your mastery over your heritage extends to being able to influence them. You can spend 5 sorcery points to bind to your service a fiend of the same type as the one you chose in your Mark of the Beast feature. The fiend must make a Wisdom save against your spell save DC + your proficiency bonus, and is bound to you upon failure. The bound fiend is an ally to you and your companions. It also shares your initiative count, but it takes its turn immediately after yours. It will obey any verbal commands you give it (no action required by you). In combat, if no commands are issued, it will use its turn to avoid danger.

The binding lasts until you lose concentration or until dispelled, upon which the bound fiend is immediately banished to its plane of origin. You can concentrate on a spell while a fiend is bound to you, however your saves to maintain concentration on both the spell and the binding are at disadvantage. If you attempt to bind another fiend while one is already bound, the bound fiend will be banished to its plane of origin. 

Level 18: Avatar of Blasphemy

At 18th level, you have fully mastered your unholy powers. As an action, you can take up a fiendish form for 1 hour or until your concentration is broken. Your appearance in this form, as well as certain abilities it grants, is based upon the type of fiend you chose in your Mark of the Beast feature.

While you are in this form, you gain the following benefits:

• You grow one size larger.
• Your concentration for your Master of the Profane feature cannot be broken.
• You are immune to being frightened.
• You gain resistance to nonmagical damage.
• You gain temporary hit points equal to 1d10 + your sorcerer level.
• If you are brought down to 0 hit points while in this form, it is dispelled explosively. Every creature within 10 feet of your body must make a Dexterity saving throw against your spell save DC. A creature takes 5d12 + your sorcerer level in the damage type of your Embraced Damnation feature on a failed save, or half as much damage on a successful one. You do not take damage from this explosion.