The Devourer

As a servant of the Devourer, you must make sacrifices often,  by killing and burning the bodies of a number of creatures equal to half your warlock level (rounded down) every week. If you fail to do this, your maximum hit points are halved, and any remaining current hit points are also halved. You also lose access to the extra spell granted to you by The Devourer. To restore your maximum hit points (but not your current hit points) and regain access to the spell, you must kill and burn a number of creature's bodies equal to the amount missed, doubled.

You also gain access to extra class features at levels 1,6, 10 and 14.

Innate Spellcasting

The Devourer gives you the innate ability to cast the spell "Circle of Death", a sixth level, once every long rest. However, the damage dealt by the spell is halved.

Dark One’s Blessing

Starting at 1st level, when you reduce a hostile creature to 0 hit points, you gain temporary hit points equal to your Charisma modifier + your warlock level (minimum of 1).

Devourer's Empowering

Once per short rest, when making a melee attack, you can gain an advantage on the attack roll, and if the attack is successful, roll an extra damage die when calculating damage.

Devourer's Tastes

Starting at 10th level, you can choose one creature type when you finish a short or long rest. You gain advantage on attacks against that creature type until you choose a different one with this feature. Both 19 and 20 counts as a critical hit against that creature type. 

Devourer of All

Starting at 14th level, when you kill a creature with an attack, you can use this feature to instantly deal 10d10 necrotic damage to any hostile creatures within 60ft of the creature you killed.

Once you use this feature, you can’t use it again until you finish a long rest.