The Soulkeeper

You have made a deal with a purveyor of souls; perhaps a generous lich, scheming devil, or a ferryman of the river styx. They have granted you powerful magical powers and great power over souls before their passing into the ethereal plane.

Death Defied

At first level you gain the following feature: When a creature you can see drops to 0 hit points you may use your reaction to suspend that creature's death for one round. The effected creature is immune to all damage and conditions and will follow your telepathic commands moving, taking actions, and reactions for you. This creature becomes incapacitated at the end of your or the effected creatures next turn, whichever comes last. You have 1 use of this ability at level 1, 2 uses at level 5, 3 uses at level 10, 4 uses at level 15, and and 5 uses at level 20. You regain all uses after completing a long rest.

Soul Armor

Also at first level you gain the following: When a creature's hit points are reduced to 0 you may use your reaction to siphon their fleeing life force to give yourself temporary hit points equal to your warlock level + your charisma modifier.

Rule Tip: If you have temporary hit points and receive more of them, you don’t add them together, unless a rule says you can. Instead, you decide which temporary hit points to keep. For more information on temporary hit points, see chapter 9 of the Player’s Handbook.

Uncanny Persistence

At sixth level you gain the following features: 

Cling to Life: When you make a death saving throw  and roll a 12 or higher you gain 1 hit point.

Ethereal Invocations: Any eldritch invocations that improve the eldritch blast cantrip can also apply to any necromancy cantrips.

Wraith Form

At tenth level you gain the following ability: When you fail your 3rd death saving throw you may have your soul fight as a spirit for one minute.

While in this form you regain all your spell slots and hit points. You are immune to all damage and conditions, can see into the ethereal plane, can pass through solid objects, and fly.

You regain use of this ability after you complete a long rest.

Soul Gorger

Starting at 14th level, you can consume the souls you’ve enslaved to heal yourself. When your death defied feature ends you can choose to consume the soul, rather than freeing
it. If you do so, you regain hit points equal to half your hit point maximum, and each creature of your choice within 10 feet of you becomes frightened of you until the end of your next turn.
Once you use this feature, you can’t use it again until you finish a long rest.