Recast in Shadow

These creaturs are the same as any other creature, but are (as the name says) recast in shadow. See the Shadow Dragon in Monster Manuel (Page 84) for more detail. They lead a cursed existance forever plagued by their fate and hunted by those who learn of it's effects. They gain the ability to mirror limited spells innately, drawing from the shadow itself to create spell like effects. They can also use the shadow to their benefit in various ways as depicted below. 

Step into Shadow

At 9th level, you gain the ability to step from one shadow into another. When you are in dim light or darkness, as a bonus action you can teleport up to 60 feet to an unoccupied space  you  an see that is also in dim light or darkness. You then have advantage on the first attack you make before the end of the turn.

Devil's Sight

You can see in the dark as if it were bright daylight out to the range of 120 feet and dim light beyond that. To the point that you could read books in pitch black of night, shadows hold no sway over your sight. You can discern darkness and shadows as translucent shading, almost like smokey figurs on glass. Figures made of shadow hiding in shadow do not gain any benefit to hide from your sight.

Manipulating Shadow

Starting at 3rd level you have the ability to create minor illusions (silent) using shadow as per the cantrip. 

One with Shadow

Your control over shadow grows to the point that your ability to blend in with shadows becomes second nature. When you are in an area of dim light or darkness you can form the shadow around yourself to hide you in perfect darkness. Those with true sight or Devil's sight or the ability to see things without vision can see through this illusion. 

Dual Plane Existance

As a reaction you can attempt to avoid damage from physical attacks/objects by sliding further into the Plane of Shadow. Roll a dexterity saving throw contested by the roll to hit or spell DC of the attack. If the cause of the damage normally does not have an attack roll or DC, the dungeon master can set a save DC appropreate for the situation.