Worn by high-ranking members of the Cult of the Dark Embrace to enhance their powers and ensure they do not deviate from their prescribed goals. The sentience is known by its wearers as Alizar the Unforgiving. 

Devour Life

Whenever you reduce a creature to 0 hit points, the amulet devours its life energy, unless it is a construct or an undead. A creature whose life energy has been devoured by Alizar the Unforgiving can be restored to life only by a wish spell.

When it devours a soul, Alizar the Unforgiving grants you temporary hit points equal to the slain creature’s hit point maximum. These hit points fade after 24 hours. As long as these temporary hit points last, you have advantage on attack rolls, saving throws, and ability checks.

If you hit an undead or another sworn to the Dark Embrace, you take 1d10 necrotic damage and the target regains 1d10 hit points. If this necrotic damage reduces you to 0 hit points, Alizar the Unforgiving devours your  life energy.  If the creature you reduce hit points to 0 is sworn to the Dark Embrace it explodes with necrotic energy as it's Dagger of the Cult of Dark Embrace overloads with a Necrotic Fireball centered on the creature. 

Life Hunter

While you wear the amulet, you are aware of the presence of tiny, medium, and large creatures within 60 feet of you that aren’t constructs or undead; though those sworn to the Dark Embrace are invisible to this sense. You also can’t be charmed or frightened.

Alizar the Unforgiving can cast the haste spell on you once per day. It decides when to cast the spell and maintains concentration on it so that you don’t have to.

Sentience

Alizar the Unforgiving is a sentient chaotic evil item with an Intelligence of 17, a Wisdom of 10, and a Charisma of 19. It has hearing and darkvision out to a range of 120 feet.

The amulet can speak, read, and understand Common, and can communicate with its wielder telepathically. Its voice is deep and echoing. While you are attuned to it, Alizar the Unforgiving also understands every language you know. With force of will and personality Alizar the Unforgiving can overpower and control its wearer to small degrees. While it can not force the wearer to go against its nature easily; it can cause small actions and influence larger actions. Alizar the Unforgiving may attempt to gain control and influence once per turn.
The wearer must make a DC18 Wisdom save to avoid the influence and control; passing grants the wearer resistance to further control until the wearer takes a long rest or 12 hours have passed. 
Actions Alizar the Unforgiving can take while influencing wearer:
Swap Targets of an Attack or Spell: target must still be a valid target and may not be an ally of the wearer
Give Disadvantage of an Attack, Spell, or Ability: target must still be a valid target
Move: may force the wearer to move 15 feet; even if movement has already been made this turn

The wearer must make a DC12 Wisdom save to avoid the influence and control; passing grants the wearer a resistance to further control until the wearer takes a long rest. 
Attack Ally: Alizar the Unforgiving may force the wearer to attack an ally of the wearer. The target must still be a valid target.
Move: may force the wearer to move up to Double the wearer's movement; even if movement has already been made this turn.

Personality

Alizar the Unforgiving speaks with an imperious tone, as though accustomed to being obeyed.

The amulet's purpose is to consume life energy and ensure that its creator's goals in the World are met. It is very upfront with the wearer about its goals since to remove the amulet is to forfeit your life energy to it. It doesn’t care whose life energy it eats, including the wielder’s. Picking up the amulet compels you to wear it with no control or thought to wearing it. 

Alizar the Unforgiving hunger for life energy must be regularly fed. If the amulet goes three days or more without consuming life energy equal or greater than the bearer, a conflict between it and its wearer occurs at the next sunset.

Notes: Bonus: Magic, Advantage: Ability Checks, Advantage: Saving Throws, Advantage: Melee Attacks, Advantage: Ranged Attacks, Damage, Buff, Combat, Bane, Sentient