Your Infernal Demon has many traits mostly obtained when created in the bowels of hell itself.

Ability Score Increase

Your Strength score increases by 5.

Age

Due to the nature of a demon they can live 1500 years or more because they tend to seek ways to increase their lifespan.

Alignment

Most demons purpose in life is to bring chaos among all. This means majority of the time this leads to them being Chaotic Evil. 

Size

Your size is Large. You stand between 8 and 12 feet tall and weight anywhere between 500 to 800lbs.

Hellish Resistance

You have resistance to fire damage.

Radiant Vulnerability

You are vulnerable to radiant damage.

Devil 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 1d12 + Your Strength Modifier slashing damage instead of the bludgeoning damage normal for an unarmed strike.

Powerful Build

You count as one size larger when determining your carrying capacity and the weight you can push, drag, or lift.

Demon Presence

You are an Expertise in the Intimidation skill and have advantage.

Languages

You can speak, read, and write Common, Abyssal, Infernal and one other choice of language

Truesight
A monster with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane within the same range.

When flying above one might mistake an abyssal devil as a medium to large sized dragon because of their wingspan. That is until they get a look at their massive bulky bodies, long fingerlike claws and horns. Abyssal devils large bodies can reach as tall as 12 feet tall with wings resembling that of a bats but larger and more durable.

Abyssal devils are naturally pretty self centered and only care about themselves. Only thing that they care about is gaining more power and extending their lifespan so they can use that lifespan to gain even more power. They also tend to use brute force or intimidation to get what they want. Most tend to force servants to work beneath them as slaves and if they do not provide what the devil needs then the devil just ends their existence.

Abyssal devils were created for one purpose, Evil. Anything they do is devious and the only purpose is to cause pain and despair like their brothers and sisters in the abyss. Very rarely do abyssal devils start in the abyss and come out anything but evil and if they do many of them lose all rights to call themselves a devil.

For a very long time devils of this nature were kept secret deep within the depths of the abyss but one day this all changed. One day a group of devils banded together to breach through the portals and reach the surface. Heavy amounts of resistance was given to try to prevent this but because of the sheer numbers and brute strength these abyssal demons held there was no chance anyone could prevent them from doing what they wanted. After succeeding they showed others the way thus releasing devils of many types to reign terror upon any who they came across.