Thanatos

Due to for example your death, a vision, or an object connected to the underworld, you meet Thanatos, the grim reaper of the Greek mythology and you strike a bargain. He will allow you to live in exchange for your service. You will wander the continents and planes on his behalf, collecting the souls of the unfortunate.

Expanded Spell List

Due to your pact with the Grim Reaper, you gain access to an expanded spell list, which help you with the tasks you were given and you will be given in the future.

1st level: False Life, Feather Fall

2nd level: Continual Flame, Phantasmal Force

3rd level: Blink, Vampiric Touch

4th level: Greater Invisibility, Phantasmal Killer

5th level: Wall of Force, Cone of Cold

True Nature

The bargain also allows you to be able to see lost souls wandering around. In order to be able to capture these souls, you get a small item (which can look like any small item you choose), which will trap souls. If you choose pact of the blade at 3rd level, this item gets substituted by the pact weapon.

Also, starting at 1st level, your patron bestows upon you the ability to project the beguiling and fearsome presence of Death. As an action, you can cause each creature in a 10-foot cube originating from you to make a Wisdom saving throw against your warlock spell save DC. The creatures that fail their saving throws are all charmed or frightened by you (your choice) until the end of your next turn.

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

In addition when choosing this Patron, you will be able to unlock alternative features for a few of the pacts and the invocations that you will choose later on.

 

Pacts:

Pact of the Blade: The weapon that you summon will allow you to cut souls from the bodies of the dead, storing them inside your pact weapon. The souls will then stay in the weapon for 1d4 days before being sent to the afterlife. While souls are in the sword, they cannot be resurrected. Your weapon deals necrotic damage.

Pact of the Chain: One of the souls you harvested breaks free or you encounter a wandering soul that takes a liking to you. You can keep this soul as your familiar. The details of this are up to the DM. The CR of this creature has to be within the range of 0-1.

 

Invocation:

Improved Pact Weapon: Feeding souls to your pact weapon makes it stronger. Having Thanatos as your patron allows you gain an additional +1 on your next attack after you kill a creature with your pact weapon.

Locate Souls

Reaching 6th level, you have become familiar with death and harvesting souls for your patron.

Once per short or long rest, you can locate lost souls. These souls have no living body to return to and wander aimlessly. You sense the direction of the soul's location and if the soul is moving, you know the direction of said movement as long as the soul is within 1,000 feet of you. This ability also works when you are trying to find a specific soul, but for this you need to know their full name and their appearance prior to their death. The ability lasts for 10 minutes.

Defying Death

By now you have harvested enough souls to earn a secure place within the ranks of Thanatos' deciples. Due to this, he grants you an extended lifespan and a few other upgrades that allow you to work more efficiently.

Beginning at 10th level, you can hold your breath indefinitely, and you don't require food, water, 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.

Etherealness

Reaching 14th level, you learn how to travel to the Ethereal Plane without the help of others.

You step into the border regions of the Ethereal Plane, in the area where it overlaps with your current plane. You remain in the Border Ethereal for the duration or until you use your action to dismiss the spell. During this time, you can move in any direction. If you move up or down, every foot of movement costs an extra foot. You can see and hear the plane you originated from, but everything there looks gray, and you can’t see anything more than 60 feet away. While on the Ethereal Plane, you can only affect and be affected by other creatures on that plane. Creatures that aren’t on the Ethereal Plane can’t perceive you and can’t interact with you, unless a special ability or magic has given them the ability to do so. You ignore all objects and effects that aren’t on the Ethereal Plane, allowing you to move through objects you perceive on the plane you originated from. When the spell ends, you immediately return to the plane you originated from in the spot you currently occupy. If you occupy the same spot as a solid object or creature when this happens, you are immediately shunted to the nearest unoccupied space that you can occupy and take force damage equal to twice the number of feet you are moved. This spell has no effect if you cast it while you are on the Ethereal Plane or a plane that doesn’t border it, such as one of the Outer Planes.

Since you are accustomed to being around dead souls and have learned their way of living and talking, you can interact with creatures that you might encounter in the Ethereal Plane. They can give you advice or warning, but they cannot give you any items.

When you used this feature, you cannot use it again until you finished a long rest.