Pact of One's Soul

Sometimes when a creature is dying, their desire to live can spark a pact between body and soul. The creature is then revived and will find that they have newfound magical abilities. But this unnatural preservation of life has its costs. Whenever magic is used part if the soul withers and the warlock must siphon the soul from another creature. They must do this every month to stay alive even without using their magic.

Life beyond death

Starting at first level, because you have defied death, you have advantage on death saving throws.

Unnatural soul: Roll 6d6 dice, in that many days you must kill a creature and siphon out its soul or else you take 1d8 necrotic damage which increases by one d8 every level.

Expanded spell list

The magic from your pact grants you additional spells

1st: False life, inflict wounds

2nd: Spiritual weapon, wither and bloom

3rd: Life transference, revivify

4th: Death ward, phantasmal killer

5th: Raise dead, reincarnate 

Ravenous soul

Starting at level 6, you can choose one creature within 100 feet and force them to make a dc 18 constitution saving throw or lose half their hit points. You either gain 1d10 hp (add 1d10 for every 2 levels you progress after 6th) or 1d8 necrotic damage to add to your next attack damage.

You can't use this again until you've taken a long rest.

Incorpreal puppet

Starting at 10th level, you can use an action to have your soul leave your body. This lasts until the soul (which has a fourth of your hit point maximum) dies, until your body's hit points are reduced to half or less, or until you return back into your body. During this time, your physical form is incapacitated and you see and move with your soul form. Your soul can pass through walls and turn invisible. When in soul form you can't attack with your spells and can only use unarmed strikes which deal force damage instead of bludgeoning. 

Master of siphoned souls

Starting at 14th level, you can call upon the souls you have siphoned to aid you, they can do one of 3 things:

Army: You cast spirit shroud at the highest level possible without expending a spell slot or material components

Possess: You can possess up to 10 creatures for an hour, in which they must do whatever you command them to. They can make a dc 15 wisdom saving throw to resist the effect

Bless: You bestow a ghostly blessing on your allies, letting them deal 2d10 necrotic damage to their attack damage.

You must take a long rest before using this feature again.