Demonic Bloodline

In your family had in a point in time had crossed paths with a fiend, maybe they had dabbled with the abyss or had made a deal with a devil, now your magic is derived from the your demonic blood

Fiend ancestry

At level 1 you choose the fiend that your family is related to, if your related to a devil you can speak write infernal, if your related to a demon you can speak and write abyssal.

Demon

Your ancestors have bang a demon or have messed with a demon, now you can speak and write abyssal 

Devil

Your ancestors have banged or made a deal with a devil, now you can speak and write infernal 

Fiend Ally's and Enemy's

At level 6,thanks to your fiendish blood, you have advantage on charisma rolls with your fiend type but disadvantage with the other and you deal double damage to the enemy fiend type.

Demon Enemies

You have advantage on charisma rolls on demons but disadvantage on devils and you deal double damage to devils 

Devil Enemies

You have advantage with charisma rolls on devils but disadvantage on charisma roll s on demons and you deal double damage against demons 

Demonic Fortitude

At level 14, your body toughens letting you to resist a elemental damage type after a short rest

Acid

You choose to resist acid damage after a short rest

Cold

You choose to resist cold damage after a short rest

Fire

You choose to resist fire damage after a short rest

Force

You choose to resist force damage after a short rest

Lightning

You choose to resist lightning damage after a short rest

Poison

You choose to resist poison after a short rest

Thunder

You resist thunder damage after a short rest

Control fiends

At level 18, you can assert your will over a fiend that is the type that you are related to, causing it to roll a DC 10 wisdom saving throw, if failed it is under your control 

Control Demon

You can force your will over a demon making it roll a DC 10 wisdom saving roll, if failed the demon is under your control 

Control Devils

You can make a devil do a wisdom DC10 saving roll, if failed they are under your control