Demon Claws

The end of your fingertips extend into very sharp claws.  These claws are treated as natural weapons that you can also make unarmed strikes with.  If you hit with them you do 1d6 + Your Strength Modifier slashing damage (for 2 limbs),  2d6 + Your Strength Modifier slashing damage (for 4 limbs),  This replaces the normal Unarmed Strike.

Demon Tail

Your demon tail is strong and as such can be used as a natural weapon.  On a hit you deal 1d8 bludgeoning damage,  on a critical hit you can also knock them prone if they fail a dc 10 Dexterity save. 

Your Nightmare form grants you the following:

Charge Attack

Moving at least 20 feet in a straight line at a creature grants you a charge attack.  With this you can knock the creature prone, 10 + your Strength Modify + Proficiency is the DC,  Versus their Dexterity or Strength save.   On a failure they are knocked prone and you then can make a Trample attack,  On a Success they are unmoved and no Trample Attack is made.

Trample

You can take a trample attack as a bonus action against a creature laying prone in front of you.  Make an attack roll and on a hit you deal 1d6 Bludgeoning damage + 1d4 Fire damage from you flaming hooves.

Powerful Build

As a large quadruped you have advantage on any save or check utilizing your form. (This includes but not limited to:  Jumping,  Pulling large loads,  Breaking through wood door/walls, etc. at the Dm's discretion.)  

Due to your Equine form you have Disadvantage on climbing checks that require Hands and Feet,  This also makes your climbing speed halved.  (But you have wings so who needs to climb!)

Illumination

At will/ your nightmare hooves shed bright light in a 10 foot radius and dim light an additional 10 feet. 

The Abyssal Nightmare Centaur's are constantly hunted by Celestials and are not accepted in normal society at all. Due to this a faction know as "The Hidden" has begun to spring up over the past few centuries.  Their only purpose is to teach hunted races and the less desirables of society how to blend in so that they can infiltrate and cause as much chaos, death and despair as possible.  If you were lucky enough to be shown the ways of The Hidden then they teach you the spell Hidden Shapechange, (A Homebrew spell) With this spell you can pass undetectable in most situations as you actually become a completely new person.