Your innate control of magic comes from the souls of mortal creatures which were bound to you at your birth. A ritual gone wrong, the sudden destruction of a soul's container, or the agonized essence of an lich or similar entity can all create a soul born sorcerer. Since soul born sorcerers are very rare, they are feared for their dark powers, even though most hosts didn't want their powers in the first place.

Bolstered Vitality

At 1st level, the power of the souls bound to you makes it harder for your life force to be drained. You gain advantage on saving throws against effects that reduce your hit point maximum, and you are resistant to necrotic damage.

Siphon Essence

When you use your magic to slay another creature, the spirits inside of you allow you to sap the power of its soul. At 1st level, when you kill one or more creatures using a sorcerer spell of 1st level or higher, you can regain sorcery points equal to your Charisma modifier (minimum of 1).

Once you use this feature, you can't use it again until you finish a short or long rest.

Soul Shield

At 6th level, you can summon the power of a soul under your control to protect yourself against harm. When a creature makes an attack roll against you,  you can use your reaction to raise a barrier of smoky energy between you and your attacker. The attack automatically misses you, and you take 1d6 necrotic damage. This damage cannot be reduced in any way. You must choose to use this feature before you know whether the attack hits or misses.

Once you use this feature, you can't use it again until you finish a long rest.

Spirit Sight

At 14th level, you gain the ability to see into the spirits of other creatures and discern their true intentions. You gain truesight out to a range of 30 feet, and you have advantage on Wisdom (Insight) checks made to tell whether a creature is lying. 

Dark Soul

Beginning at 18th level, you can channel the full power of the spirits within you, creating an aura of life draining energy. As an action, you can spend 6 sorcery points to draw on this power and exude an aura of desolation to a distance of 30 feet. For 1 minute or until you lose your concentration (as if you were casting a concentration spell), each hostile creature that starts its turn in this aura must make a Constitution saving throw, taking 2d10 necrotic damage on a failed save, or half as much on a successful one. In addition, while maintaining concentration you are immune to nonmagical damage and have advantage on all ability checks and saving throws.