Your soul is tainted with the energies of the Abyss and you draw you magic from it. The abyssal energy slowly grows within you warping both your magic and very soul. When the energy has completely enveloped your soul you will become a demon, that is the fate of the tainted. Many of the tainted seek to cleanse themselves of the Abyss's touch, whereas others embrace this strange new power granted to them.

There are a few ways one can become an abyss touched. For example a abyssal Tiefling may have been born with a strange excess of abyssal energies, or you where near a demonic incursion in your youth and while you got away, you weren't unscathed.

Eyes of the Abyss

You have darkvision with a range of 120ft. Your darkvision is not based on light but granted via abyssal magic, hence you can still see in total darkness, but not magical darkness.

Abyssal constitution

 You gain the following resistances at the corresponding levels

Cold resistance

Starting at 3rd level you gain cold resistance

Fire resistance

Starting at 1st level you gain fire resistance 

Poison resistance

Starting at 5th level you gain poison resistance and advantage on saving throws against being poisoned

Demonic summoning

Starting at 6th Level you learn the summon lesser demons spell which doesn't count against the total number of spells learnt. You can cast it with a spell slot or 5 sorcery points, if cats with sorcery points the demons will not be hostile towards you.

Starting at 7th Level you learn the summon Greater demons spell which doesn't count against the total number of spells learnt. You can cast it with a spell slot or 6 sorcery points, if you cast the spell with sorcery points the summoned demon will not try to break free of your control and will obey all spoken commands. regardless of how the spell was cast you can use spell slots or sorcery points to boost the Cr of the summoned demon up to a total Cr equal to your max spell slot level plus 1. It costs 2 sorcery points per Cr level increased when booted with sorcery points.

Demonic mutations

Pick one of the options listed.

demonic wings

gain a flight speed of 30ft

Fiendish charm

You  gain proficiency in persuasion and add your CHA modifier to your AC

Improved Abyssal Constitution

You become resistant to non magical bludgeoning, piercing and slashing damage.

Improved demon summoning

When you cast the summon greater demons spell you can spend sorcery points to raise the Cr of the summoned demon up to equal your sorcerer level, it costs 2 sorcery points for every additional level up too 10, and 3 sorcery points for every level above 10. Additionally you learn the planar binding spell.

Demonic essence

When you die in any plane other than the abyss your body disintegrates into ashes and you re-spawn in the abyss along with all your equipment.

Your body now rejects the touch of healing of divine magic, healing spells now damage you and regular resurrection spells have no effect on you, instead fire heals you by half the total damage. You also gain the Fiend tag and the abyss is counted as your native plane, this means that you are effected by some spells that usually wouldn't effect you (dispel good and evil, magic circle, banishment etc)

All resistances granted by previous class features become immunities but you now gain a vulnerability to radiant damage.

You now have a demonic true name, if someone issues a command to you using your true name and you resist you must pass a DC 18 Wis Save or take 8D8 psychic damage. If you write your true name on an amulet made of abyssal metals and perform a secret ritual (up to DM's discretion) it becomes a demon amulet containing a portion of your demonic essence. If ever you die in the abyss you can re-spawn at any time or place on the same plane as the demon amulet. If the amulet is ever destroyed however you are trapped in the abyss for 101 days, only a spell of ninth level or higher can release you, and you cannot be the one who cast the spell.