Tenebris Witch

They're born from a form of dark magic, their magic becoming darker more connections to witches, and it is common to have one of these become masters of dark magic.  They are quite common to appear in the fey wild created by witches and hags.

Dark Sorcery

Starting at 1st level, you can learn any warlock cantrips or a 1st level warlock spell, they don’t count towards your sorcerer cantrips known or sorcerer spells known but you can only use this spell once per long rest.

Alchemist Witchery

Starting at 1st level, You can use alchemist's supplies or potion as a spellcasting focus, if you use this as a spellcasting focus if the spell you cast with it has a damage roll you can add 1d4+your proficiency bonus necrotic damage to the roll, and you can use this as a spellcasting focus even if it doesn’t have a material component.

Dark Love

Starting at 6th level, you are able to use your dark magic to charm any creature.  As a reaction to an attack or you use your action, you can stop the attack if you used a reaction, and charm them for 2 turns if they fail your spell save dc, once after every long rest.

Witch Curse

Starting at 14th level, you are a master at the dark magic of curses and can curse someone at will.  You can cast bestow curse at will and does not use any spell slots you do not need concentration and it lasts for 5 hours you can only cast this once every long rest. 

Tenebris Sorcery

Starting at 18th level, you have reached your full potential and have mastered the art of dark magic and have the ability to twist fate.  When a creature makes a saving throw from one of your spells they have disadvantage on the saving throw if they fail the saving throw they deal an extra d6 equal to the spell's level necrotic damage.