Darkblade rogues manipulate the very shadows to suit their needs. Shadows become cloaks, portals, illusions, and even tools.

Knightshade

In becoming a Darkblade, Death has decided to show you his favor. Your very shadow becomes your most stalwart companion, a Knightshade, providing you with boons to evade detection, escape injury, and strike down your foes. On your turn as a bonus action, you can change the form of your Knightshade and it will act in that capacity until directed otherwise.

Shadow Serration

Your blades are enveloped in freezing shadows. Any weapon damage you do is augmented by Cold Damage. You gain proficiency in Death's weapons.

When you attack a target that is in Dim Light or Darkness, you make the attack roll with Advantage and add additional 1d8 Cold Damage.

When you attack with Advantage, Critical Hits require one less number on your attack roll. This effect can stack.

Shadowstrider

Shadowstride. As a Bonus Action you can teleport up to double your remaining movement range to an unoccupied point you can see for the cost of half of the traveled distance. If you start and end in Dim Light or Darkness, it is a free action. 

Knightshade: Nightcloak

You have advantage on Stealth checks when you are Lightly Obscured or Heavily Obscured.

When you are Heavily Obscured at the end of your turn, you automatically become Invisible until your next turn.

You cannot have disadvantage on Stealth checks.

Your Knightshade can take the form of moving shadows, giving you and nearby allies +10 to Stealth checks as with Pass without Trace. Alternatively, it can become a cloud of Darkness which does not affect your vision.

Knightshade: Phantom Limb

Your Knightshade can take the form of a Shadow Hand, which can interact with the world and improve your combat skills.

Knightshade: Shadowclone

Your Knightshade takes the form of... you. This acts like the Mirror Image spell or Invoke Duplicity, giving you extra evasive capabilities and advantage on attack rolls. If you are successfully hit this defense drops, but on your next turn the Shadowclones will reform.

Knightshade: Reaper's Mark

Your Knightshade takes the form of a curse mark. Choose a creature you can see and mark it for death. The number someone needs to roll for a Critical Hit when attacking this creature is reduced by 3. This effect can stack. The marked creature also automatically fails any Stealth checks against you and you know their exact location at all times. You always have Advantage while attacking the marked creature.

When the marked creature is killed, its killer refreshes their action. After the target is killed, the mark transfers to the next creature you attack. The mark lasts until short rest or until you alter the form of your Knightshade.