Shadow Knight's Training.

Fighters touched by the plane of shadow itself reign supreme in the dark. These warriors, called Shadow Knights, cloak themselves in darkness to become elusive phantoms on the battlefield. With each strike, they unleash the dark and chilling powers of the realm of shadow, sowing fear and uncertainty in the hearts of their enemies. While evil often thrives in darkness, Shadow Knights have no singular motivation, and simply use the powers of darkness as they would a sword or shield.

Level 3 Features:

Dark Gaze.

When you choose this archetype at 3rd level, you're touched by the plane of shadow. You can see normally in dim light and darkness both magical and nonmagical to a distance of 60 feet.

Manifest Shadow.

Starting at 3rd level, you can use a bonus action to manifest an object made of solid darkness, known as shadowstuff, in your hand. If the object isn't a weapon, it can be no larger than 1 foot on a side and weigh no more than 5 pounds, and its form must be that of a nonmagical object that you've seen.
If the object is a weapon, you can choose the form that it takes, although it must be a melee weapon. You are proficient with it while you wield it. Its damage die is a d8 (adding your Strength or Dexterity modifier to the damage roll as normal) or a d10 if you wield it with both hands, and it deals psychic damage to any creature it hits; the weapon is harmless against objects and structures. The weapon has the finesse, light, and thrown properties (range 20/60), and when you use it to hit a nonmagical light source (such as a lamp or torch), the light is extinguished. In addition, when you use the weapon to attack a target that's in dim light or darkness, you make the attack roll with advantage, and attacking it at long range doesn't impose disadvantage on the roll.
If you drop the shadow object or throw it, it dissipates at the end of the turn. Otherwise, the object dissipates when you use this feature again or when you command it to dissipate (no action required). You can use a bonus action to cause the object to reappear in your hand.

Shadowcasting.

Also at 3rd level, you can create special magical effects using shadowstuff. If the effect requires a saving throw, it uses a DC of 8 + your Wisdom modifier + your proficiency bonus.
-Shadowbite: Immediately after hitting a target with a shadow weapon, you can force it to make a Constitution saving throw. The target has disadvantage on the saving throw if you had advantage on the attack roll against it. On a failed save, the target takes an extra 1d8 psychic damage, and the next attack roll it makes before the end of your next turn is made with disadvantage.
-Umbral Coating: You can use a bonus action to coat a physical weapon you're holding in shadowstuff. For the next hour, that weapon has the thrown property (range 20/60), and you can use a bonus action to magically return the coated weapon to your open hand. Furthermore, the weapon counts as a shadow weapon for the purposes of triggering any of the features from this subclass.
You can use this feature a number of times equal to your proficiency bonus. You regain all expended uses of it whenever you finish a short or long rest.

Level 7 Feature:

Umbral Warrior.

By 7th level, armor you wear no longer imposes disadvantage on your Dexterity (Stealth) checks, and you have advantage on Dexterity saving throws while in dim light or darkness.

Level 10 Feature:

Improved Shadowcasting.

Starting at 10th level, whenever you use an action to use a Shadowcasting option, you can use a bonus action to make a single weapon attack with a shadow weapon you're holding. In addition, you gain the following three Shadowcasting options:
Cloak of Shadow: You can use an action to choose up to four creatures that you can see within 20 feet of you. For the next hour, those creatures have advantage on Dexterity (Stealth) checks.
Darkness: You can use an action to cast the Darkness spell.
Eyes of the Dark: You can use a bonus action to choose up to four creatures that you can see within 20 feet of you. For the next hour, those creatures gain the benefits of your Dark Gaze feature.

Level 15 Feature:

Shadow Sneak.

Starting at 15th level, whenever you hit a target with a shadow weapon, you can immediately teleport to an unoccupied space within 5 feet of it and turn invisible. You remain invisible until the start of your next turn or until you attack or cast a spell. Once you use this feature, you can't do so again until you finish a short or long rest.

Level 18 Feature:

Cover of Darkness.

By 18th level, the shadows you take refuge in protect you: you're considered to have half cover whenever you're in dim light or darkness.