Your Charisma score increases by +2 (terror is, technically, a Charismatic effect) and your Constitution increases by +1, thanks to your immortal heritage.

Devil's Remnants can live for centuries - though they normally die a violent death long before then.  They reach maturity at around 30 years.

Devil's Remnants are very rarely Lawful, putting protection of friends and family far above the law.  They can be any of Good, Neutral, and Evil.

Devil's Remnants are 6.5 to 8 feet tall.  They weigh 200-300 pounds.  Your size is Medium.

Your base walking speed is 30 feet; in equine form, your flying speed is 30 feet.

You have resistance to fire damage.

Devil's Remnants cannot gaze upon the light of heaven.  Whenever you take radiant damage, you are blinded unti the start of your next turn.

Starting at 3rd level, you can use your action to unleash the demonic energy within yourself. Your eyes either glow (if white) or seem to suck in the light around them (if black).  The shrieks of the damned emanate from the air around you, and the sky grows darker.

You surround yourself with an Aura of Fear.  The instant you do this, other creatures within a 10 foot radius sphere centered on you must each succeed on a Charisma saving throw (DC 8 + your proficiency bonus + your Charisma modifier) or become frightened of you until the end of your next turn.  The Aura does not cause the frightened effect again after its initial activation.

The Aura of Fear lasts for 1 minute or until you end it as a bonus action. During it, once on each of your turns, you can deal extra psychic damage to one target when you deal damage to it with an attack or a spell. The extra psychic damage is equal to your Charisma modifier.

Once you use this trait, you can’t use it again until you finish a long rest.

Starting at 3rd level, as an action you can transform into a unicorn with massive, iridescent, feathered wings.

While you are in this form, beginning the instant you transform, you are surrounded by an Aura of Awe that extends from you in a 10 foot radius sphere.  Any creature entering the Aura or within it when it activates must succeed on a DC 14 Charisma saving throw or become charmed by you until either they successfully make a Charisma saving throw or your transformation ends, dissipating the Aura.  Charmed creatures roll a new Charisma saving throw at the end of each of their turns.

In this form, you gain a flying speed of 30 feet and the attack listed below. Your ability score for STR becomes 18, for DEX becomes 12, and for CON becomes 15, but your CHA, INT, and WIS scores do not change.  Your hit point maximum is: 4 + (your level times 8).  However, you do not gain hit points onto your current total when you transform.  Your walking speed increases to 60ft.  You cannot speak while transformed, but can understand both Common and Celestial.  You cannot make any weapon attacks other than Impale or cast spells with a somatic or verbal component while transformed. You can maintain concentration on spells.  You do not get any bonus to AC from your armor.  Because of the positioning of your wings, you cannot be ridden by a medium creature.

Impale. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 2d6+4 piercing damage.

Charging Impale. If the alicorn moves at least 20 feet straight toward a creature and then hits it with an impale attack on the same turn, the target takes an extra 2d6 piercing damage.  The target must also succeed on a DC 14 Dex saving throw or be stuck impaled on the horn, subject to a grapple.  They can attempt to escape the grapple on subsequent turns using the standard checks.

(Note: you can use the homebrew monster Angelic Alicorn for reference.  The only differences are your hit point maximum and your CHA, INT, and WIS scores.)

The transformation lasts one minute unless you dismiss it as a bonus action.  After you have transformed back, you cannot enter your angelic form again until you have competed a long rest.

Starting at 6th level, your Angelic Form can be accessed again after completing a short rest.

In your humanoid form, you can speak Common and Infernal.  In your equine form, you cannot speak but can understand both Common and Celestial.

Before his Fall, your ancestor, Amduscias, led the great angelic choir.  His celestial voice rings in your own.  You gain proficiency in Performance and have advantage on Charisma checks against any creature that has heard you sing for at least a minute within the last hour.  They do not need to be consciously aware that it was you singing.  For instance, you could sing from the other side of a curtain from a creature for a minute, come through the curtain a few minutes later, and have Charisma check advantage against the creature without it knowing that you were the person singing minutes earlier.