You bore witness to horrors that your mind simply could not resolve. Your psyche has created a series of logical loopholes in order to protect itself. You have learned to control and manipulate your warped perception of reality in order to manifest nothingness into the world.

Null Blade

At 3rd level, you have learned how to pull the shadow of your weapon free from the physical material, like a sword from its sheath. This Null Blade is made of pure shadow and deals Necrotic damage instead of its original type. The Null Blade counts as magical for the sake of overcoming resistances and can only be used on 1 weapon at a time. If the weapon ever leaves your grasp, voluntarily or not, it ceases to exist.

Peer into the Void

Starting at 3rd level, You have stared into the dark for so long that it no longer holds horrors for you. You gain the following benefits:

You gain Darkvision of 60', or extend your DarkVision by 60' if you already have it.
You have advantage on checks and saves against Fear and/or Intimidation effects. 
You gain proficiency in Intimidation checks, or double the proficiency bonus if you are already proficient.

Purge Light

At 7th level, darkness encroaches on your vision. You have found that you can manifest that darkness into the world around you. You can see through Magical darkness as if it were regular darkness.

As a Bonus Action, manifest a 10' sphere of magical darkness in a space within range of your Darkvision. These spheres of darkness last for 10 minutes. You can summon a number of these spheres equal to your proficiency modifier per short rest.

Shared Delusion

At 10th level, most of your traditional vision is gone. You perceive the world through degrees of shadow and blackness, regardless of the light around you.

You are considered blind and are immune to effects requiring sight.
You gain Blind Sight to the range that your Darkvision would normally be, and benefit from spells or items that extend the range of Darkvision.

Additionally, any zone of darkness that you are touching or that you create is considered to be normal darkness to your allies.

Reality is fluid

At 15th level, your understanding of reality has become so strained, that you frequently forget how physics works.
You can "Swim" in areas of Magical Darkness as if they were water, but you use your normal movement speed as the swim speed for those endeavors.

Call of the void

Starting at 18th level, any magical darkness you create or are influencing is regarded as the safest possible place available. Any dangers that are in the affected zones of darkness are treated as safe to creatures you do not consider to be an ally. If a creature knows of a danger that the magical darkness is obscuring, they have to make a Wisdom Save versus your Intimidation or Persuasion Check to treat the area appropriately. Creatures that make a successful save are immune to this effect until the following sunrise.