Your Dex increases by 3.
Your Con increases by 2.
Your Cha increases by 1.

Unknown. However some contractors have re order that their demons, though few as they were, had lived well passed 3000... If they weren't... Reckless.

We... Are demons. We aren't... All monsters, but most of us are...

Your alignment is normally true chaotic. While neither good or bad, evil or lawfull, chaos is most certainly what you are. 

Upon creation take a Chaotic alignment unless your backstory allows for another.

You are medium for the reason of "having a human like skinsuit" That you shift in and out of. 

Your base walking spees is 35 ft. 

As an elemental "look-alike" demon/fiend entity you may "produce" demon wings that allow 60 ft of fly speed. Though it's more like strange hover/gliding.

You know Common, Infernal, Abysmal and Telepathy link up to 120ft

You gain racial proficiency in Arcana, Deception, Insight and Stealth.

Because you are not human, or rather than you've transcended the existence of the mortal plane, you have insanely monstrous levels of abilities.

Con, Dex, Wis maximums increase to 24.

Note: And because it's a monster to a "Controlable" NPC race. Con, Dex, and Wisdom have a racial score maximum of 24. 

While you immune to necrotic, you are vulnerable ro radiant. 

You are also resistant to Acid, Cold, Fire, Lightning, Poison; Bludgeoning, Piercing, and Slashing from Nonmagical Attacks

Lastly you are immune to these conditions: Exhaustion, Grappled, Paralyzed, Petrified, Poisoned, Prone, Restrained, Blinded.

You have Darkvision-120ft, blindsight- 60ft

Claws. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: 10 (2d6 + 3) psychic damage or, if the demon had advantage on the attack roll, 17 (4d6 + 3) psychic damage.

Multiattack. The demon makes two claw attacks.

Possession (Recharge 6). One humanoid that the demon can see within 5 feet of it must succeed on a DC 14 Charisma saving throw or be possessed by the demon; the demon then disappears, and the target is incapacitated and loses control of its body. The demon now controls the body but doesn't deprive the target of awareness. The demon can't be targeted by any attack, spell, or other effect, except ones that turn, dispel, or banish fiends, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.

The possession lasts until the body drops to 0 hit points, the demon ends it as a bonus action, or the demon is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the demon reappears in an unoccupied space within 5 feet of the body. The target is immune to this demon’s Possession for 24 hours after succeeding on the saving throw or after the possession ends.

A Dark Shadow of 8th level or higher can vanish from his current location and reappear in any shadowy area within 90 feet. This is a move-equivalent action.

While in dim light or darkness, the demon can take the Hide action as a bonus action.

Shadows are difficult to hit in shadow, and as a result gains a +2 to their armor class while in darkness.

The Dark Shadows innate spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). The Dark Shadows can innately cast the following spells, requiring no material components:

At will: detect thoughts, disguise self, mage hand, minor illusion

3/day each: charm person, detect magic, invisibility, major image, suggestion

1/day each: dominate person, fly, plane shift, true seeing

The fiend magically enters the Ethereal Plane from the Material Plane, or vice versa.