The Void Beyond

 You have made a pact with the Void Beyond, a cosmic abyss of sentient darkness revered by the Obsidian Apostles. This entity—perhaps a shard of the goddess of darkness herself or a remnant of the War of Shadows—offers power through surrender, annihilation, and rebirth in shadow.

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Embrace the Black Tar

 As a bonus action, you cause inky tendrils to erupt from your form for 1 minute. Once per turn, when you hit a creature with a spell or pact weapon attack, you can deal an additional [roll]1d6[/roll] necrotic damage and reduce its movement speed by 10 feet until the start of your next turn.

You can use this feature a number of times equal to your Cha bonus, and regain all uses after a long rest.

Darkness isn't the boonies

Tarstep: can move 30 feet as a bonus action through Black Tar regions 
Tarvision: any movement in Black Tar regions within 30 meters of warlock reveal moving thing and give advantage on attacks against anything moving

Dark Insight

You gain proficiency in Wisdom saving throws. Additionally, whenever you succeed on a Wisdom saving throw, you can teleport up to 10 feet to an unoccupied space you can see as a reaction, leaving behind a flicker of your shadowy form.

Shroud of the Apostles

While you are in dim light or darkness, you have resistance to all damage except force and radiant. When a creature within 30 feet of you takes psychic or necrotic damage, you may use your reaction to heal yourself for half the amount dealt.

Avatar of Eternal Dusk

As an action, you can transform for 1 minute. While in this form:

• You are under the effects of greater invisibility.
  
  
• Your necrotic damage ignores resistance.
  
  
• When you reduce a creature to 0 hit points, you may choose to summon a Wisp of Dread (use shadow - Legacy).
  
  

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

Book of Void

The following spells are added to the warlock spell list for you:

Spell Level	Spells
1st	cause fear, arms of Hadar
2nd	mirror image, silence
3rd	hunger of Hadar, counterspell
4th	greater invisibility, phantasmal killer
5th	dream, cloudkill