The Phylactery

Twisted by the dark magic of a lich, or those seeking to become one, the sorcerer finds themselves as either a willing or unwilling living phylactery.

Knowledge of the Grave

When making any kind of check that involves creatures of death, your proficiency bonus is doubled for that check, regardless of any other skill in that area.

Half Life

Your body is accustomed to energy of death, granting a resistance to necrotic damage. 

Undead Control

Beginning at 6th level, you can use a sorcery point to force your will on a creature of the undead. As a bonus action, choose one undead creature. That creature must succeed on a wisdom saving throw. If it fails, you gain complete control over it for 1 hour, at which point the control ends, and the creature turns to ash. Undead that succeed gain immunity to this spell for 24 hours. Furthermore, creatures with Legendary Actions have immunity to the affects.

Death's Door

Beginning at 6th level, whenever you kill a creature, their soul enters into the phylactery. You can hold a number of souls equal to your charisma modifier. You can use it to fuel a variety of magic.

Beyond the Grave: You expend a soul to cast Speak With dead once a long rest. You can only speak with the soul that was used in the casting, but they must succeed on a charisma saving throw. On a failed save, they must answer any question you ask truthfully. If the soul succeeds, the spell functions otherwise normally, beyond not needing a body to be cast. 

Life From Death: You can expend 3 souls as a bonus action to restore HP equal to your sorcerer level.

Necrotic Overload: You can expend 5 souls to and cause a necrotic spell to deal maximum damage, ignoring resistance and treating immunity as resistance. If used, you cannot use any Meta Magic options for 3 rounds. The spell functions as normal otherwise. 

Necrotic Rot

Beginning at 18th level, you can channel the necrotic rot within you. As an action, you can spend 5 sorcery points and choose a number of targets equal to your charisma modifier. Each target must succeed on a constitution saving throw. On a fail, the targets take 5d8 necrotic damage, and their hit point maximum is permanently reduced by that same amount, unless a Greater Restoration or Wish spell is cast to reverse the affect. You gain half the health lost. On a successful save, the target takes half damage and suffers no other effect.