The Blood

In the search for power, you made a pact with a powerful being of death, either willingly or on accident. This being may be a deity, such as Bhaal, Myrkul or Raven Queen, or a feind such as the Demon Lord Orcus. In place of a normal pact, this patron has imbued you with an Ichor, a mass of pure dark energy. The Ichor takes your body as its host, establishing a permanent connection between you and your patron. In exchange for draining your body of its life force, the Ichor will allow you to cast extremely powerful spells. In exchange for granting you power, the being of death charges you with his/her will: to spread death in his/her name. 

The Ichor is a dark mass of pure energy. It is usually black with a secondary color such as dark red or purple. When the Ichor bonds with you, it enhances your physical appearance to your whim. When it is not contained your body, it takes the form of a weightless fluid suspended in the air, constantly moving and shifting. The Ichor cannot be damaged or destroyed, and only a wish spell can break the bond between the Ichor and you. It is a construct of your patron, and allows you to commune instantaneously with him/her. The Ichor also fuels your vitality, allowing you to gain an additional 1d12 hit points per warlock level.     

The Ichor draws on your life blood in order to cast spells. In return, the spells are stronger, and at later levels, have additional effects. In order to cast cantrips, you must expend 1 hit point. Casting 1st and 2nd level spells require 2 hit points. Casting 3rd, 4th, and 5th level spells require 7 hit points. Casting 6th, 7th, and 8th level spells require 16 hit points. Casting 9th level spells require 23 hit points. All spells that would do damage do an additional 1d4 damage starting at the 1st level, 1d6 starting at the 7th level, 1d8 starting at the 13th level, and 1d10 starting at the 17th level, and 1d12 starting at the 20th level.  

Your patron also allows you to affect all the actions you take. By expending 10 hit points, you can add an additional 1d10 to any ability check or attack roll you make. You can only do this three times per short or long rest.

Blood Rebate

Starting at the 1st level, your patron will reward you for the death you cause. For any creature you kill, you will receive temporary hit points equal to 1/5 of the creature's maximum hit points (rounding up) + your Charisma modifier. That becomes 1/4 at the 3rd level, 1/3 at the 11th level, and 1/2 at the 18th level.  

Eldritch Rite

Starting at the 6th level, your patron will imbue your eldritch blast cantrip with additional effects.    

Eldritch Chain

Your eldritch blast seeks to destroy all who stand in its path. Upon a successful hit, your blast will arc to another available target within 10 feet. You must roll to hit and damage again. If there are no available targets within range, the chain ends. The blast can chain a maximum of 3 times.  

Focused Ray

Your eldritch blast is concentrated into a potent beam. You have advantage to hit.  

Paralyzing Lance

Your eldritch blast is charged with static energy, rendering the target paralyzed for 1 round.    

Vampiric Siphon

Your eldritch blast draws the life force of your targets away from them. Upon a successful hit, you gain temporary hit points equal to 1/2 the damage you inflicted.    

Dark Commune

Starting at the 10th level, your patron will indulge your inquiries in exchange for a sacrifice. Once per long rest, immediately after you have killed a creature, you may cast the commune spell, without expending a spell slot. Due to the fiendish nature of your patron, they will occasionally provide a false answer to your question.      

Avatar of Wrath

Starting at the 14th level, you can mold the Ichor to summon a Blood Knight. Once per day, by expending half of your maximum hit points, you can expel part of the Ichor from your body in the form of a dark reflection of yourself. The Blood Knight has hit points equal to those you expended, and has the same equipment as you, formed of crystallized Ichor. A telepathic bond between you and the Blood Knight allow you to command its actions. The Blood Knight cannot cast spells or gain temporary hit points, but otherwise has the same traits as you. Upon death, the Blood Knight's body explodes, dealing 3d8 necrotic damage to all creatures within a 5 feet of it. If the Blood Knight does not die, it will despawn after a number of minutes equal to your level. You can take an action to dispel the Blood Knight early.