The Reaper

Your patron is a grim reaper, valkyrie, shinigami, or other spirit responsible for the taking of dead souls to the afterlife. Such beings are often wearied by their eternal existence, and their motives range from fulfilment of an age-old contract to merely finding some alleviation of the tedium of eternity. Source: Eldritch Expension by Toby Lowther.

(optional) Scythes

The scythe, as well as the sickle, have long been represented as the weapon of choice by which psychopomps "harvest" the souls of the fallen.

If your Warlock of the Reaper wishes to use a scythe, you may use the general statistics for the glaive as presented in the Player's Handbook. Alternatively, at the DM's discretion, you may use the following option:

Scythe. Martial Melee Weapon. 2d4 slashing damage. Heavy, reach, special, two-handed.
Special. If you hit a creature with an attack with the scythe, you can attempt to move that creature up to 5 feet in any direction except away from you. That creature can avoid this effect by making a successful Strength (Athletics) or Dexterity (Acrobatics) check (their choice), contested by your Strength (Athletics) check.

Expanded Spell List

The Reaper lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

1st dissonant whispers, false life
2nd blur, gentle repose
3rd feign death, speak with dead
4th death ward, phantasmal killer
5th antilife shell, legend lore

Twilight Soul

Starting at 1st level, your agreement allows you to tred the fine line between life and undeath. When damage reduces you to 0 hit points, you can make a Constitution saving throw (DC equal to 5 + the damage taken), unless the damage was radiant or from a critical hit. On a success, you drop to 1 hit point instead.

You can use this feature a number of times equal to your Charisma modifier (a minimum of once). You regain all expended uses when you finish a short or long rest.

Reaper's Path

At 6th level, your patron grants you access to their secret passage. As a bonus action, you can use this feature to turn invisible and become insubstantial. You produce no sound or scent, and cannot be detected by nonmagical means. You are not invisible or undetectable to creatures with 1 hit point or fewer, although you appear spectral and enveloped in shadows to them.

You can move through other creatures and objects, including walls and closed doors, as if they were difficult terrain. If you end your turn inside an object, you take 1d10 force damage.

While this feature lasts, you cannot attack, cast spells, or use any magic item or feature that would interact with another creature or object. You see the world in shades of gray.

This feature lasts a number of turns equal to your Charisma modifier (a minimum of 1), ending at the end of your turn. You can choose to end this feature immediately as a bonus action.

If the feature ends while you are inside a solid object (such as a wall), you immediately take 5d10 force damage, you are restrained and incapacitated, and you cannot breathe.

Once you have used this feature, you cannot do so again until you finish a short or long rest.

Half-Life

Beginning at 10th level, you do not need to breathe, and you do not require food, drink, or sleep, although you still require rest to reduce exhaustion and still benefit from finishing short and long rests.

In addition, you age at a slower rate. For every 10 years that pass, your body ages only 1 year, and you are immune to being magically aged.

Mark of the Reaper

When you reach 14th level, when you hit a creature with a spell attack or a creature fails a saving throw against a spell you cast which effects only that creature, you can use this feature to mark that creature for the grave.

If you attempt to mark a creature which does not have a soul, the use of this feature is spent but it has no effect.

The mark lasts for a number of days equal to your Charisma modifier (a minimum of 1). As long as the mark lasts, you can determine that creature's direction and distance from you if it is on the same plane of existance as you, or which plane it is on if it is on a different plane of existance, by spending 1 minute meditating and consulting your patron.

While the mark lasts and the creature is on the same plane of existance as you, you can use an action to end the mark, attempting to rip the creature's soul from its body. The creature must make a Wisdom saving throw against your spell save DC. On a failed save, that creature drops to 0 hit points. On a success, it takes 10d10 necrotic damage. An undead creature makes this saving throw with advantage.

You can have only one creature marked by this feature at a time. You can choose to end the mark harmlessly without using an action. Once you use this feature, you can't use it again until you finish a long rest.