Darkin

Based on a book series called NeverNight, this aims to give the thief some new shadow inspired abilities, giving it a pseudo caster type feel, while remaining themed. Control of the shadows is really up to the DM when it comes down to it, and a lot listed here is general ideas. As a player grows in levels, I hope to convey that they are able to control the shadows more and more.

Touched by the Shadows

Your character was born with a unique gift: The Shadows obey you. Your control over them is tenuous at best, but with time and practice, you might one day be able to shroud the world in darkness. In the meantime, you only know you are blessed because of your lifelong companion. A Darkin meets their companion early on in their life, and it takes the form of a loved creature, pet, or animal that they knew. This companion is immune to all attacks, spells, and effects, and only speaks what languages you know, in addition to Shadow-Tongue, the language of these creatures. Your companion is a reflection of your person, and takes after its master in almost every way. Your companion lives in your Shadow, and consumes your fear, leaving you fearless.

Shadow Worker

Starting at 3rd level, you can use a bonus action to control the shadows of the room and hamper your enemies or aid yourself.

You claw at their eyes, working the shadows around them to blind them to the room. Your enemy is blinded until the end of it's turn, or can use a bonus action to shake off the effect.

You wrap their feet in the shadows pooling around them, causing them to trip and stumble. Your opponent is unable to take a movement action until the effect is bypassed, or removed using a bonus action on their turn.

You are able to aid yourself in the fight, using the shadows in the room to bring you advantageous objects. You can call an object to you using the shadows under your control, as long as it is not under the control of an enemy or unwilling ally.

You are able to create a number of thievery tools using shadows, allowing you to pick locks with seemingly nothing, and escape from your bonds with no apparent help. You are able to use the shadows instead of lockpicks for picking locks, and can add your dexterity modifier to the roll.

Shadow Cloak

You weave the shadows around your frame, blending you into your surroundings, and making yourself nearly invisible to those around you. While cloaked in this way, you have advantage on all stealth checks, and are granted double your proficiency and dex mod to the roll. While hidden in this way, you only have vision within 5 ft of you, and outside of this range only see vague shapes and blurs. You have disadvantage on any dexterity checks made while cloaked in this way.

At levels 11 and 17, you are able to cloak allies that are touching you. Up to 2 other medium creatures at 11th and 4 medium creatures at 17th.

Shadow Walker

You have practiced long enough at working the shadows to your will that you are now able to thrust yourself between two shadows. Up to 2 times per short rest or long rest, you are able to step into a shadow and exit anywhere within eyesight, as long as a shadow is there. You are also able to step more than 2 times, but will suffer a point of exhaustion for each time used past your second.

Dark Drinker

If you slay another Darkin, and are the one who landed the killing blow, you consume their abilities, permanently bolstering your own. You are granted an additional 2 uses of all class abilities per Darkin slain, and are able to work the shadows easier and quicker than ever before.

You are now also able to Step Between an additional 2 times per short or long rest, before suffering from exhaustion.

Shadow of a Dead God

Your mastery over the shadows is nearly complete, and few are able to contest your control. Your debilitating effects now take a full action to remove, and you are able to Step Between the shadows a total of 6 times before suffering exhaustion. Your Shadow Cloak now no longer blinds you, and your control over shadows allows you to manipulate locks and items from range with ease, allowing you to pick locks, trigger traps, and grab objects from anywhere in eyesight. You also have BlindSense within 200 ft. surrounding you on anything touching a shadow.

Your companion riding in your shadow is now corporeal to only you, and can be pet. Yes, you can now pet your companion. If that isn't a worthy 17th level reward I don't know what to tell you gentlefriends.