demon shell

you have a demon trapped inside you and you can't quite control the magic that pours from it through you. You not necessarily evil but your fragmented mind can be compelled to harm people every once in a while. The magic you use unleashes the demon at the moment of the spell. After each spell you cast, you need to roll a constitution saving throw = 20 - 3 + con mod and sorcerer level. If you fail the saving throw then the demon pushes out and causes a surge of foul magic to rise and has one small to medium-lasting effect that will dissipate after one hour or till reversed.

demon soul

pick the demon that has a piece of itself in you and gain magical abilities from it

Baphomet/Demogorgon/Marilith/Balor

You know the thaumaturgy cantrip. When you reach 3rd level, you can cast the hellish rebuke spell as a 2nd-level spell once with this trait and regain the ability to do so when you finish a long rest. When you reach 5th level, you can cast the darkness spell once with this trait and regain the ability to do so when you finish a long rest. Constitution is your spellcasting ability for these spells.

fiendish control

you learn to wrangle the demon inside of you and can push it down after you cast magic.

you can spend sorcery points to push the demon inside of you back down.
1 sorcery point = +2 to saving throw, the score increases plus two for each sorcery point spend above one

also, increase your constitution score plus one to a max of 20

demon body/demon ward

your relationship with the demon will either increase the chaos that emanates from you or you can use the demon to master it.

Demon Body: the essence of your demon seeps more and more into your body and once a long rest you can let the demon out and increase the spell slot you are using by four. When you use this feature you automatically fail your demon saving throw.

Demon Ward: you learn how to look within yourself and can converse with the demon that resides in you. You make a pact with this demon and it starts to talk to you and you can talk to it. Your relationship with the demon decreases the DC for the demon saving throw by 3.

Inner Unity

you gain the ability to control the demon better and can use its power to a further extent, but it will still force its way out from time to time.

once per long rest you can merge with your demonic aspect and power yourself up. Every spell you cast has its spell level increases by 3 and you gain a plus 2 to your armor class.

Every demon saving throw you would make you automatically fail and if you start your turn with half or fewer hit points you will need to make a demon saving throw.