The Unknown Shadow

You made a pact with an evil creature who empowers others to achieve their goals. They aren't infernal in any way, which sets them apart from fiends. These beings are often in search of universal conquest, and have connections between their alternate selves. Such beings are elusive, and are never known by their true name, though they may give others something to refer to them by.

Dark knowledge

Starting at 1st level, you know what goes on in the world, and how to take advantage of it. You gain half proficiency in Arcana, Deception, History, Religion, and Sleight of Hand.

Shadow Mantle

Starting at 6th level, you cast no shadow, as it protects the outside of your body. This feature has one charge. If you have one or more charges of this feature, you gain resistance to bludgeoning, piercing, and slashing damage from nonmagical attacks. Taking radiant damage expends one charge of this feature. All charges are restored at the end of a short or long rest.

Twisted Form

Starting at 10th level, your patron twists and changes your body in unnatural and disturbing ways. Choose a mutation when you gain this feature. All mutations are permanent and can only be reversed by a wish spell.

Amphibious

You gain slits in your neck to act as gills, in addition to webbing on your hands and feet. These supernatural gills provide you with the ability to breathe when submerged in any liquid. The webbed appendages give you 30ft of swim speed, and allow you to ignore rough terrain while fully submerged.

Exoskeleton

Your bones leave the inside of your body, only to form a powerful shell on the outside. You gain resistance to piercing, slashing, fire, and acid damage, as well as a +3 to your AC. In return, you also gain vulnerability to bludgeoning damage.

Gemstone Claws

Your hands form into large crystalline claws that give you better grip, and serve as a powerful weapon. You decide the type of gem upon taking this option. The type is merely cosmetic. You gain a climbing speed of 30ft and a burrowing speed of 10ft, as your claws can dig into almost any surface. You also can make a melee weapon attack that deals 3d8 slashing damage + 2d4 necrotic damage.

Large Tail

Your legs meld together into one long, large, and scaly tail. As a result, your movement speed is halved, but you also gain immunity to rough terrain and being prone. You also gain 30ft of swim speed. In addition, you can make a melee weapon attack that deals 2d8 bludgeoning damage.

Polyped

Your lower body starts extending backwards, as your legs get smaller, and you grow several new ones behind them (You look kind of like a centipede). You gain 30ft of walking speed, 60ft of climbing speed, an immunity to being prone, and you grow one size larger. Any armor or clothing you are wearing at the time will adjust to your new form, but other nonmagical armor or clothing will have to be custom made for your body.

Spines

You gain spines reminiscent of a porcupine on your back. These spines are extremely long, making 5ft of space directly behind you rough terrain, costing 4ft of movement speed to move 1ft, or 2ft for every 1ft and taking 2d8 piercing damage. Anyone who melee attacks you from behind takes 2d8 piercing damage. In addition, you can make a ranged weapon attack with your spines to deal 2d8 damage to up to 3 targets.

Telekinetic Hands

Your arms and legs disappear, leaving you with only hands left. These hands act like mage hand, but with the same functionality as a normal hand. This gives you an extra 25ft of melee range, making things like swords much more viable against enemies with long ranged weapons. You also have no legs, so you can hover up to 5ft in the air. 

Wingspan

Your arms form into full fledged wings that are still capable of acting as hands. You gain 30 ft of flight speed and can only be knocked out of the air by certain conditions or magical attacks. The appearance of the wings are up to you. They will generally have a darker coloration.

Broken Mind

Starting at 14th level, your mind becomes twisted an broken. Deception becomes exceptionally easy due to a distorted morality. Despite this, your alignment does NOT change. You also learn multiple different languages. These include: Abyssal, Deep Speech, Infernal, Primordial, and Undercommon. In addition, you gain the ability to know any creature near flawlessly simply by observing them.