Demonic Summoner

Your innate magical nature stems from the Infinite Abyss itself. This rare ability most frequently derives from an ancestor who had mastered conjuration magics, specifically harnessing powerful demons. Sometimes, this demonic power originated from the influence of a mighty demon's touch on your heritage, whether through a patronage or even an instance of demonic possession. Either way, the power to summon forth and harness demons and their strength runs through your veins.

Abyssal Communion

You can speak, read, and write Abyssal. In addition, you gain the ability to speak Abyssal telepathically to any creature who also understands the language.

At 6th level, you gain the spell Summon Lesser Demons, and at 9th level, you gain Summon Greater Demons. Neither of these spells count against your known Sorcerer spells.

Blood of the Demons

Starting at 1st level, your demonic blood grants you abilities akin to those of the demon race. You gain darkvision for a total of 60 feet. In addition, your unnatural blood prevents the affects of poison, making you immune to poison damage and the poisoned condition.

Fiendish Harness

At 6th level, you gain the ability to control the demons your power originated from. When you encounter or summon forth a demon, you now have the power to overtake their will and substitute your own. In order for this to take place, you must spend ten minutes ritually tethering one demon of your choice to your being. Once the demon has been targeted, a black pentagram matching the size of the demon appears underneath it. The demon must be kept within the restraints of the pentagram for the entire ten minute duration, or the ritual fails. At the end of the ritual, the demon must make a wisdom saving throw versus your spell save DC. If the demon is not presently hostile (I.E. unconscious or charmed), then it has disadvantage on said saving throw. On a success, the ritual has no effect. On a fail, the demon's will becomes tethered to your own, granting you compete control over it. A demon remains under your control until it dies, or is released from your will by another use of the ten minute ritual. 

A harnessed demon acts under your initiative. You may forgo one of your attacks on an Attack action for a demon to take an action. 

The maximum challenge rating of a demon you can control is equal to your demonic willpower (half your sorcerer level (rounded down)). Multiple demons can be controlled at once as long as the sum of their challenge ratings does not exceed your demonic willpower. The maximum number of demons that can be harnessed at once is equal to your Charisma modifier.

Harnessed demons can be brought to and from the Abyss at will. Using an action, any one of your harnessed demons can be brought into existence through a black pentagram emblazoned on a nearby surface. The demons remain present until you return them to the Abyss with another action, or death.

Abyssal Vigor

At 14th level, the demonic blood inside you boils with increased demonic power whenever you have a demon summoned. As long as one of your harnessed demons in within 60 feet of you, you gain resistance to cold, fire, and lightning damage, as well as advantage on saving throws against any spells or magical effects.

In addition, your innate magical abilities grant you further dominion over demons. You can now add half of your Charisma modifier (rounded down, minimum of 1) to your demonic willpower level. In addition, when your demons fall to 0 hit points, they instead fall to 1 hit point and immediately return to the Abyss. They remain harnessed to you, although they cannot be summoned again until the end of your next long rest. 

Power of the Infinite

Starting at 18th level, you are able to fuel yourself with the untapped power of the Infinite Abyss. Whenever you summon forth a demon, you gain 30 temporary hit points. If an additional demon is summoned, your temporary hit points reset to 30.

In addition, you can now add your full Charisma bonus to your demonic willpower (minimum of 1).