The Master Thief

A true genius of theft and subtlety, a thief so talented, they stole the power of a god and still no one knows who they are. The Master Thief is your patron. The Master Thief acts through a special knife that only you can summon, which grants you the talents and abilities any thief would need to accomplish whatever the job requires.

Dusk Knife

At 1st level, by utilizing and expending a handful of metal equal to or greater than 1gp of worth, you summon a special knife made of solidified shade, as an action;
this is the Dusk Knife. It is a simple melee weapon with which you are proficient. The Dusk Knife deals 1d4 psychic damage on a hit and has the finess, light and thrown properties (range 20/60). When you use the attack action with the Knife, you can use your Charisma modifier, instead of Strength or Dexterity, for the attack and damage rolls. The Dusk Knife may be used to pick locks and disarm traps as if you were using and had proficiency with Thieves Tools, using your charisma modifier in place of Dexterity.

If you drop or throw the Dust Knife, you can use a bonus action to cause the Dust Knife to reappear in your hand. If it is taken or lost, it must be resummoned using new materials and the previous Knife disappears after being resummoned.
At higher levels, The Dust Knife becomes more powerful. When you take the attack action to use your Knife, the number of d4s rolled increases as you gain warlock levels, increasing to 2d4 at level 4, 3d4 at level 6, 4d4 at level 10, 5d4 at level 14 and 6d4 at level 18.

If you later gain the Pact of the Cloak feature, once per turn when you attack with your Dusk Knife and roll with advantage or with an ally within 5 feet of your target, you may add 1d8 plus your warlock class level to the damage.

Thief's Intuition

At 1st level, your patron imparts some of their grace, choose either Sleight of Hand or Stealth to gain proficiency in. 
Additionally, prior to making a skill check or saving throw that you don't have proficiency in, you may use your reaction to add your proficiency bonus to the roll.
This feature can be used a number of times equal to your Charisma modifier, and all expended uses are restored when you finish a short or long rest.

Thief's Talents

At 6th level, your patron grants you abilities they knew in life.
You learn Thieves Cant, as per the Rogue feature.
Additionally you may allow your ghostly patron to guide you for the next minute (no action required), you gain Thief's Talents.
While you have Thief's Talents, you have advantage on Dexterity saving throws; additionally once on each of your turns you may use on of the following abilities:
Shadow Rope: Cast levitate or Misty Step without expending a spell slot which lasts untill the end of your next turn.
Subtle Arts: When you cast a spell, you may cast it without any somatic or verbal component.
Honed Reflexes: As a bonus action, take or plant an item on a humanoid or perform the Disengage, Hide or Search action.
Once you have used this feature, you may not use it again unless you have taken a long rest, or have expended a warlock spell slot to use it again.

Armor of Shade

At 10th level, your Dusk Knife creates a protective cloak of shadows around you. After taking the Hide action or when rolling an attack with advantage while using your Dusk Knife, you gain temporary hit points equal to half your warlock level.

Master Thief

At 14th level, your patron can help you in the subtle arts, as well as your companions. When you or an ally you can see within 30 feet is making a skill check, prior to the roll, you may use your reaction to give advantage to the roll. You may use this feature a number of times equal to your proficiency bonus per long rest.
Additionally, whenever you or a willing creature you can see rolls a 1 on a Dexterity check or Dexterity saving throw, you may use your reaction to reroll the skill check or saving throw, taking the new roll instead.