Reaper

You create a pact with the reaper of souls. This Patron is always around, escorting the souls of those who have passed to the after life.  Taking no sides in any conflict, they are true Neutral. 

Soul Crystal

When you reach 1st level the Reaper bestows upon you a Soul Crystal. This is used to ferry souls to the after life. When a creature dies within 30 feet of you, you can use your reaction to absorb the soul into a crystal. You can absorb a number of souls equal to your warlock level. These souls can be used in a number of ways. your range increases to 60 ft at level 10 and 120feet at level 17

1. Soul Shield as a reaction you expend 5 souls from your crystal to create a necrotic shield in a 10 foot radius that gives a +2 AC to every creature you chose in the area. Additionally when a creature who you didn't chose starts its turn or enters the shield for the first time they take 1d8 necrotic damage.

2. Soul Bomb as an action you expend all remaining charges to charge a necrotic bomb. On your next turn your the Bomb goes off, and every creature within the 30 foot radius makes a dex  save or takes 1d12 necrotic damage per charge on a failed save and half as much on a successful save including the caster. Once you use this feature you cant use it gain until you finish a long rest. If your concentration is lost, you lose the souls as they escape the crystal with no effect. Each 

3 Eldritch Soul When you cast Eldritch Blast you can expend 5 souls to release an additional blast to the attack.

 4 Healing Soul You Expend 2 souls from the crystal and cast Healing Word at 1st level

 5 Soul Siphon when you kill a creature you can steal a spell slot from the creature by using 2 souls from your crystal for every level of spell slot  you are stealing.

At the end of a long rest all remain charges are released to the Reaper, in exchange he gives you 2 temp HP per soul remaining in the Crystal.  when you use any of your other features that use a soul when the creature dies you don't get to absorb the soul into the crystal 

Control Souls

Starting at 6th level, when a creature with in 30 feet of you dies, you collect its soul and use its energy on your next spell/melee  attack to give you advantage. the range increases to 60ft at Level 10 and 120 feet at level 17

You can use this feature a number of times equal to your CHA modifier per long rest

Soul Tap

Starting at 10th level, you can add a d10 to any saving throw made with in 30 feet of you. You then take that d10 roll and multiply it by 2 and take that amount of necrotic damage. 

Relentless Soul

Starting at 14th level, when you kill a creature you can use this feature to cast another cantrip/spell or make an Melee attack.