Somewhere down the line you've came into contact with a being from Shadowfell. It left a mark on you, that spreads on your body over time.
The mark sharpened your senses and now aids you in ending other beings. 

Those afflicted by this mark have danced with death often and become accustomed to it. As a result their empathy towards other beings sufferings has diminished.

Felltouched Features

Shadowsight

When you choose this archetype at 3rd level, your mark grows, tinting the skin around your eyes black.
The mark grants you darkvision up to a range of 60 feet.
If you already posess darkvision, its range increases by 60 feet. This can only increase your darkvision range to a maximum of 150 feet, after all other effects were taken into account.

Shadow Blade

Starting at 3rd level, you become able to make a shadow blade appear at will. The blade uses your Dexterity modifier.
It deals 1d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60). In addition, when you use the blade to attack a target that is in dim light or darkness, you make the attack roll with advantage.
You have one charge, that you can use as a Bonus Action to channel darkness into the blade, increasing its damage by 1d8 per charge for 1 minute.
All charges replenish during a short rest.

If you drop the weapon or throw it, it dissipates at the end of your turn. You can make the blade reappear in your hand.
If you hold an item in the hand you make the shadow blade appear in, that item will be dropped.

You gain addtional charges at level 9, 13 and 17 to a maximum of 4 charges.

Umbral Step

At 9th level the darkness spreads to your limbs, tinting the skin along your veins black.
Your advanced transformation enables you to use a bonus action to teleport through the shadows cast by creatures that you can see within 120 feet.
The space, on which the shadow is cast, has to be unoccupied.
If you see a creature that is within 120 feet and in darkness you can teleport to any free space within 5 feet of that creature.

Emit Darkness

At 13th level you become more accustomed to the darkness caused by the mark. You become able to emit darkness around you. Emitting darkness reduces the visible effects that the mark has on you.
As an action, you can create an area of darkness centered on you that covers an area of 30 feet and lingers for 2 Minutes. Creatures with darkvision other than you can't see through it. 
Light, other than daylight, is not able to illuminate the area.

You are able to use your umbral step at any target that is located within the darkness.
Once you've used this feature, you can't use it again until you have finished a long rest.

From the Depths

At 17th level you become hard to defeat. When damage that isn't radiant would reduce you to 0 hit points, you instead recover half your maximum hit points. When this happens, the visible effects the mark causes on your body increase.
For the next minute your body will emit shadowy smoke that creates an area of 15 feet centered on you, which dims light. The smoke appears monstrous to any observer.
In this area, you can see, as if it was illuminated. Light sources can not illuminate this area or dispell the effect.

During this effect, you can call upon two shadow blades.  
If you have any expended charges for your shadow blade you will regain half of them (rounded up). Charges used to channel darkness will effect both shadow blades.
You wont be able to maintain a second shadow blade after the end of the effect. You can choose which shadow blade vanishes.

This can't happen more than once per long rest.