Umbral Stalker

A halfling moves through the shadows unnoticed by the crowd, an orc strikes their target quick and swiftly, leaving cloaked by the darkness of the night as quick as they arrived, a shadowy projection of a dwarf appears below a rich man and robs him blind without him ever noticing its presence. Umbral stalkers are legends among thieves, those that move so swiftly and silently that legends of supernatural powers shroud them, and they might not just be legends after all...

Thieve’s Shadow

When you choose this archetype at 3rd level, you come into contact with the umbral energies between the planes of existence, your shadow gains life and follows your commands, it has the same status as a Shadow (MM. 269), but it has half your Hit Points, it's considered a neutral aligned fey creature instead of an undead, its Strength Drain attack is replaced by a claw attack that deals 2d4 Psychic Damage, it cannot be charmed and you may telepathically communicate with it for up to 120 feet.

The Shadow has its own initiative and if it's Hit Points are reduced to zero it reappears on you, only being able to disconnect again after a short or long rest, while the Shadow is disconnected, as an action you may see through its senses.

Penumbral Retribution

At 3rd level, whenever a creature makes a melee attack against you, hitting or missing, you may use your reaction to retaliate at them, three shadow copies of you encircle the creature, dealing deadly blows to it, the creature must make a Constitution Saving Throw or take Psychic Damage equal to your sneak attack damage and get frightened of you until the end of your next turn, on a save the creature takes half damage and isn't frightened. You may use this ability once per long rest.

 The DC for this ability is equal to 8 + Proficiency Bonus + Intelligence Modifier.

Hidden Knives

Reaching 9th level, you learn to harness the umbral power between the planes to create daggers from shadow itself, as a bonus action you can create up to two daggers made from shadow, they deal 2d4 Psychic damage and dissipate after 1 minute or until you dismiss them, if thrown they return automatically to your hand at the end of your turn.

Shadow Step

At 13th level you learn to travel through the shadows unnoticed, twice per long rest as a bonus action, you may teleport up to 30ft to an unoccupied space, if you move into melee with a creature, you may make a melee attack right after, if you move away from any creature, you gain advantage on your next Stealth (Dex) check for one minute. While in dim-light or complete darkness the range of this ability is doubled fo 60ft.

Wraith Aspect

Once you reach 17th level you push your body to the limit between the planes, once per long rest, your body becomes shadowy and semi-incorporeal, for 10 minutes you gain the following abilities:

Smoke Form. You gain flight speed equal to your movement and resistance to all non-magical damage.

Incorporeal Movement. You can move through other creatures and objects as if they were difficult terrain. You take 5 (1d10) force damage if you end your turn inside an object.

Improved Knives. The Hidden Knives damage goes up to 2d6.